Support building with -Werror=strict-prototypes / -Werror=old-style-definition

Unfortunately "-std=c99" is not sufficient to make gcc ignore code that
uses constructs of earlier C standards, which were abandoned in C99.

See https://lwn.net/ml/fedora-devel/Y1kvF35WozzGBpc8@redhat.com/ for
some related discussion.

Change-Id: I84fd99442d0cc400fa562fa33623c142649230e2
diff --git a/tests/gsm0502/gsm0502_test.c b/tests/gsm0502/gsm0502_test.c
index f4f6530..e9deaa9 100644
--- a/tests/gsm0502/gsm0502_test.c
+++ b/tests/gsm0502/gsm0502_test.c
@@ -86,7 +86,7 @@
 	502782, 502825, 502869, 502903, 502955, 502999
 };
 
-static void test_gsm0502_fn_remap()
+static void test_gsm0502_fn_remap(void)
 {
 	unsigned int i;
         uint32_t fn_begin;