selftests/bpf: Fix compiler warnings in bpf_testmod for kfuncs

Add -Wmissing-prototypes ignore in bpf_testmod.c, similarly to what we
do in kernel code proper.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/oe-kbuild-all/202304080951.l14IDv3n-lkp@intel.com/
Link: https://lore.kernel.org/bpf/20230412034647.3968143-1-andrii@kernel.org

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -28,6 +28,10 @@ struct bpf_testmod_struct_arg_2 {
 	long b;
 };
 
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+		  "Global functions as their definitions will be in bpf_testmod.ko BTF");
+
 noinline int
 bpf_testmod_test_struct_arg_1(struct bpf_testmod_struct_arg_2 a, int b, int c) {
 	bpf_testmod_test_struct_arg_result = a.a + a.b  + b + c;
@@ -175,6 +179,8 @@ noinline int bpf_testmod_fentry_test3(char a, int b, u64 c)
 	return a + b + c;
 }
 
+__diag_pop();
+
 int bpf_testmod_fentry_ok;
 
 noinline ssize_t