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/src/dgsm.c b/src/dgsm.c
index bfa5df8..cdd82a2 100644
--- a/src/dgsm.c
+++ b/src/dgsm.c
@@ -191,7 +191,7 @@
 	dgsm_mdns_client_config_apply();
 }
 
-void dgsm_stop()
+void dgsm_stop(void)
 {
 	g_hlr->mslookup.allow_startup = false;
 	mslookup_server_mdns_config_apply();