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/gad/gad_test.c b/tests/gad/gad_test.c
index ca8c4fa..19be6a6 100644
--- a/tests/gad/gad_test.c
+++ b/tests/gad/gad_test.c
@@ -4,7 +4,7 @@
 #include <osmocom/core/msgb.h>
 #include <osmocom/gsm/gad.h>
 
-void test_gad_lat_lon_dec_enc_stability()
+void test_gad_lat_lon_dec_enc_stability(void)
 {
 	uint32_t lat_enc;
 	uint32_t lon_enc;
@@ -49,7 +49,7 @@
 	},
 };
 
-void test_gad_enc_dec()
+void test_gad_enc_dec(void)
 {
 	int i;
 	printf("--- %s\n", __func__);
@@ -115,7 +115,7 @@
 	}
 }
 
-void test_gad_to_str()
+void test_gad_to_str(void)
 {
 	int i;
 	printf("--- %s\n", __func__);
@@ -134,7 +134,7 @@
 	}
 }
 
-int main()
+int main(int argc, char **argv)
 {
 	test_gad_lat_lon_dec_enc_stability();
 	test_gad_enc_dec();