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/smscb/cbsp_test.c b/tests/smscb/cbsp_test.c
index d530798..2dbdded 100644
--- a/tests/smscb/cbsp_test.c
+++ b/tests/smscb/cbsp_test.c
@@ -81,7 +81,7 @@
 	return msg;
 }
 
-static void test_decode()
+static void test_decode(void)
 {
 	struct msgb *msg;
 	struct osmo_cbsp_decoded *cbsp_dec;