Replace '.' in counter names with ':'

The '.' is illegal character in counter names, as they are exported
via CTRL interface, where '.' has a special meaning that cannot be
used by strings comprising the variable name.

Change-Id: I4eb6851d3577f5942ae1a9ab28d3b3ca7ab8e208
diff --git a/src/gprs/gtphub.c b/src/gprs/gtphub.c
index 7e3e663..5e962f2 100644
--- a/src/gprs/gtphub.c
+++ b/src/gprs/gtphub.c
@@ -125,7 +125,7 @@
 };
 
 static const struct rate_ctr_group_desc gtphub_ctrg_io_desc = {
-	.group_name_prefix = "gtphub.bind",
+	.group_name_prefix = "gtphub:bind",
 	.group_description = "I/O Statistics",
 	.num_ctr = ARRAY_SIZE(gtphub_counters_io_desc),
 	.ctr_desc = gtphub_counters_io_desc,