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/include/osmocom/core/counter.h b/include/osmocom/core/counter.h
index 0d56bc4..7b677cb 100644
--- a/include/osmocom/core/counter.h
+++ b/include/osmocom/core/counter.h
@@ -52,7 +52,7 @@
 
 int osmo_counters_for_each(int (*handle_counter)(struct osmo_counter *, void *), void *data);
 
-int osmo_counters_count();
+int osmo_counters_count(void);
 
 struct osmo_counter *osmo_counter_get_by_name(const char *name);