samples/bpf: Remove unnecessary -I flags from libbpf EXTRA_CFLAGS

Commit [0] breaks samples/bpf build:

    $ make M=samples/bpf
    ...
    make -C /path/to/kernel/samples/bpf/../../tools/lib/bpf \
     ...
     EXTRA_CFLAGS=" \
     ...
     -fsanitize=bounds \
     -I/path/to/kernel/usr/include \
     ...
    	/path/to/kernel/samples/bpf/libbpf/libbpf.a install_headers
      CC      /path/to/kernel/samples/bpf/libbpf/staticobjs/libbpf.o
    In file included from libbpf.c:29:
    /path/to/kernel/tools/include/linux/err.h:35:8: error: 'inline' can only appear on functions
       35 | static inline void * __must_check ERR_PTR(long error_)
          |        ^

The error is caused by `objtree` variable changing definition from `.`
(dot) to an absolute path:
- The variable TPROGS_CFLAGS is constructed as follows:
  ...
  TPROGS_CFLAGS += -I$(objtree)/usr/include
- It is passed as EXTRA_CFLAGS for libbpf compilation:
  $(LIBBPF): ...
    ...
	$(MAKE) -C $(LIBBPF_SRC) RM='rm -rf' EXTRA_CFLAGS="$(TPROGS_CFLAGS)"
- Before commit [0], the line passed to libbpf makefile was
  '-I./usr/include', where '.' referred to LIBBPF_SRC due to -C flag.
  The directory $(LIBBPF_SRC)/usr/include does not exist and thus
  was never resolved by C compiler.
- After commit [0], the line passed to libbpf makefile became:
  '<output-dir>/usr/include', this directory exists and is resolved by
  C compiler.
- Both 'tools/include' and 'usr/include' define files err.h and types.h.
- libbpf expects headers like 'linux/err.h' and 'linux/types.h'
  defined in 'tools/include', not 'usr/include', hence the compilation
  error.

This commit removes unnecessary -I flags from libbpf compilation.
(libbpf sets up the necessary includes at lib/bpf/Makefile:63).

Changes v1 [1] -> v2:
- dropped unnecessary replacement of KBUILD_OUTPUT with $(objtree)
  (Andrii)
Changes v2 [2] -> v3:
- make sure --sysroot option is set for libbpf's EXTRA_CFLAGS,
  if $(SYSROOT) is set (Stanislav)

[0] commit 13b25489b6 ("kbuild: change working directory to external module directory with M=")
[1] https://lore.kernel.org/bpf/20241202212154.3174402-1-eddyz87@gmail.com/
[2] https://lore.kernel.org/bpf/20241202234741.3492084-1-eddyz87@gmail.com/

Fixes: 13b25489b6 ("kbuild: change working directory to external module directory with M=")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/bpf/20241203182222.3915763-1-eddyz87@gmail.com

samples/bpf/Makefile

@@ -146,13 +146,14 @@ ifeq ($(ARCH), x86)
 BPF_EXTRA_CFLAGS += -fcf-protection
 endif
 
-TPROGS_CFLAGS += -Wall -O2
+COMMON_CFLAGS += -Wall -O2
-TPROGS_CFLAGS += -Wmissing-prototypes
+COMMON_CFLAGS += -Wmissing-prototypes
-TPROGS_CFLAGS += -Wstrict-prototypes
+COMMON_CFLAGS += -Wstrict-prototypes
-TPROGS_CFLAGS += $(call try-run,\
+COMMON_CFLAGS += $(call try-run,\
 	printf "int main() { return 0; }" |\
 	$(CC) -Werror -fsanitize=bounds -x c - -o "$$TMP",-fsanitize=bounds,)
 
+TPROGS_CFLAGS += $(COMMON_CFLAGS)
 TPROGS_CFLAGS += -I$(objtree)/usr/include
 TPROGS_CFLAGS += -I$(srctree)/tools/testing/selftests/bpf/
 TPROGS_CFLAGS += -I$(LIBBPF_INCLUDE)
@@ -162,7 +163,7 @@ TPROGS_CFLAGS += -I$(srctree)/tools/lib
 TPROGS_CFLAGS += -DHAVE_ATTR_TEST=0
 
 ifdef SYSROOT
-TPROGS_CFLAGS += --sysroot=$(SYSROOT)
+COMMON_CFLAGS += --sysroot=$(SYSROOT)
 TPROGS_LDFLAGS := -L$(SYSROOT)/usr/lib
 endif
 
@@ -229,7 +230,7 @@ clean:
 
 $(LIBBPF): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OUTPUT)
 # Fix up variables inherited from Kbuild that tools/ build system won't like
-	$(MAKE) -C $(LIBBPF_SRC) RM='rm -rf' EXTRA_CFLAGS="$(TPROGS_CFLAGS)" \
+	$(MAKE) -C $(LIBBPF_SRC) RM='rm -rf' EXTRA_CFLAGS="$(COMMON_CFLAGS)" \
 		LDFLAGS="$(TPROGS_LDFLAGS)" srctree=$(BPF_SAMPLES_PATH)/../../ \
 		O= OUTPUT=$(LIBBPF_OUTPUT)/ DESTDIR=$(LIBBPF_DESTDIR) prefix= \
 		$@ install_headers