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-auc-gen.c b/utils/osmo-auc-gen.c
index 72f1fcd..e3e1b43 100644
--- a/utils/osmo-auc-gen.c
+++ b/utils/osmo-auc-gen.c
@@ -85,7 +85,7 @@
 	.algo = OSMO_AUTH_ALG_NONE,
 };
 
-static void help()
+static void help(void)
 {
 	int alg;
 	printf( "-2  --2g\tUse 2G (GSM) authentication\n"