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: I861be39275b40c522c02f553074c5a4489e87127
diff --git a/tests/auc/auc_test.c b/tests/auc/auc_test.c
index 6bf2eb2..61cf82f 100644
--- a/tests/auc/auc_test.c
+++ b/tests/auc/auc_test.c
@@ -509,7 +509,7 @@
 }
 
 /* Test a variety of invalid authentication data combinations */
-void test_gen_vectors_bad_args()
+void test_gen_vectors_bad_args(void)
 {
 	struct osmo_auth_vector vec;
 	uint8_t auts[14];