[statistics] Make counter_db_store internal

Do not expose DB functionality on this level
diff --git a/openbsc/src/statistics.c b/openbsc/src/statistics.c
index 3429d6e..4cc281d 100644
--- a/openbsc/src/statistics.c
+++ b/openbsc/src/statistics.c
@@ -56,7 +56,7 @@
 	talloc_free(ctr);
 }
 
-int counters_store_db(void)
+static int counters_store_db(void)
 {
 	struct counter *ctr;
 	int rc = 0;