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/sms/sms_test.c b/tests/sms/sms_test.c
index 9ca83a5..912c082 100644
--- a/tests/sms/sms_test.c
+++ b/tests/sms/sms_test.c
@@ -215,7 +215,7 @@
 	},
 };
 
-static void test_octet_return()
+static void test_octet_return(void)
 {
 	char out[256];
 	int oct, septets;