kbuild: fix kernel/bounds.c 'W=1' warning

Building any configuration with 'make W=1' produces a warning:

kernel/bounds.c:16:6: warning: no previous prototype for 'foo' [-Wmissing-prototypes]

When also passing -Werror, this prevents us from building any other files.
Nobody ever calls the function, but we can't make it 'static' either
since we want the compiler output.

Calling it 'main' instead however avoids the warning, because gcc
does not insist on having a declaration for main.

Link: http://lkml.kernel.org/r/20181005083313.2088252-1-arnd@arndb.de
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/bounds.c

@@ -13,7 +13,7 @@
 #include <linux/log2.h>
 #include <linux/spinlock_types.h>
 
-void foo(void)
+int main(void)
 {
 	/* The enum constants to put into include/generated/bounds.h */
 	DEFINE(NR_PAGEFLAGS, __NR_PAGEFLAGS);
@@ -23,4 +23,6 @@ void foo(void)
 #endif
 	DEFINE(SPINLOCK_SIZE, sizeof(spinlock_t));
 	/* End of constants */
+
+	return 0;
 }