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/src/counter.c b/src/counter.c
index cbee7b9..dace15f 100644
--- a/src/counter.c
+++ b/src/counter.c
@@ -75,7 +75,7 @@
 
 /*! Counts the registered counter
  *  \returns amount of counters */
-int osmo_counters_count()
+int osmo_counters_count(void)
 {
 	return llist_count(&counters);
 }