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/utils/osmo-ns-dummy.c b/utils/osmo-ns-dummy.c
index 0af1a89..eabe008 100644
--- a/utils/osmo-ns-dummy.c
+++ b/utils/osmo-ns-dummy.c
@@ -43,7 +43,7 @@
 	.copyright	= vty_copyright,
 };
 
-static void print_help()
+static void print_help(void)
 {
 	printf( "Some useful options:\n"
 		"  -h	--help			This text\n"