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/hlr_db_tool.c b/src/hlr_db_tool.c
index b2a26dc..058a12c 100644
--- a/src/hlr_db_tool.c
+++ b/src/hlr_db_tool.c
@@ -51,7 +51,7 @@
 	.db_upgrade = false,
 };
 
-static void print_help()
+static void print_help(void)
 {
 	printf("\n");
 	printf("Usage: osmo-hlr-db-tool [-l <hlr.db>] [create|import-nitb-db <nitb.db>]\n");