selftests/bpf: Fix uprobe_multi compilation error

When building selftests, the following was seen:

uprobe_multi.c: In function ‘trigger_uprobe’:
uprobe_multi.c:108:40: error: ‘MADV_PAGEOUT’ undeclared (first use in this function)
  108 |                 madvise(addr, page_sz, MADV_PAGEOUT);
      |                                        ^~~~~~~~~~~~
uprobe_multi.c:108:40: note: each undeclared identifier is reported only once for each function it appears in
make: *** [Makefile:850: bpf-next/tools/testing/selftests/bpf/uprobe_multi] Error 1

...even with updated UAPI headers. It seems the above value is
defined in UAPI <linux/mman.h> but including that file triggers
other redefinition errors.  Simplest solution is to add a
guarded definition, as was done for MADV_POPULATE_READ.

Fixes: 3c217a1820 ("selftests/bpf: add build ID tests")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240926144948.172090-1-alan.maguire@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/uprobe_multi.c

@@ -12,6 +12,10 @@
 #define MADV_POPULATE_READ 22
 #endif
 
+#ifndef MADV_PAGEOUT
+#define MADV_PAGEOUT 21
+#endif
+
 int __attribute__((weak)) uprobe(void)
 {
 	return 0;