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/ctrl.c b/src/ctrl.c
index f101d25..11b0f5c 100644
--- a/src/ctrl.c
+++ b/src/ctrl.c
@@ -749,7 +749,7 @@
 	return 1;
 }
 
-static int hlr_ctrl_cmds_install()
+static int hlr_ctrl_cmds_install(void)
 {
 	int rc = 0;