prandom: Include <linux/percpu.h> in <linux/prandom.h>

<linux/percpu.h> include was removed from <linux/prandom.h>
in d9f29deb7f ("prandom: Remove unused include") because
this inclusion broke arm64 due to a circular dependency
on include files.

__percpu tag is defined in include/linux/compiler_types.h, so there
is currently no direct need for the inclusion of <linux/percpu.h>.
However, in [1] we would like to repurpose __percpu tag as a named
address space qualifier, where __percpu macro uses defines from
<linux/percpu.h>.

The circular dependency was removed in ddd8e37ebaa1 ("random: Do not
include <linux/prandom.h> in <linux/random.h>") and it cleared
the path for the inclusion of <linux/percpu.h> in <linux/prandom.h>.

This patch is basically a revert of d9f29deb7f
("prandom: Remove unused include").

[1] https://lore.kernel.org/lkml/20240812115945.484051-4-ubizjak@gmail.com/

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

include/linux/prandom.h

@@ -10,6 +10,7 @@
 
 #include <linux/types.h>
 #include <linux/once.h>
+#include <linux/percpu.h>
 #include <linux/random.h>
 
 struct rnd_state {