bpf: Fix build warning regarding missing prototypes

Fix build warnings when building net/bpf/test_run.o with W=1 due
to missing prototype for bpf_fentry_test{1..6}.

Instead of declaring prototypes, turn off warnings with
__diag_{push,ignore,pop} as pointed out by Alexei.
Signed-off-by: Jean-Philippe Menil <jpmenil@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200327204713.28050-1-jpmenil@gmail.com

net/bpf/test_run.c

@@ -114,6 +114,9 @@ static int bpf_test_finish(const union bpf_attr *kattr,
  * architecture dependent calling conventions. 7+ can be supported in the
  * future.
  */
+__diag_push();
+__diag_ignore(GCC, 8, "-Wmissing-prototypes",
+	      "Global functions as their definitions will be in vmlinux BTF");
 int noinline bpf_fentry_test1(int a)
 {
 	return a + 1;
@@ -149,6 +152,7 @@ int noinline bpf_modify_return_test(int a, int *b)
 	*b += 1;
 	return a + *b;
 }
+__diag_pop();
 
 ALLOW_ERROR_INJECTION(bpf_modify_return_test, ERRNO);