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/gb_proxy.c b/src/gprs/gb_proxy.c
index 17a0109..6a9bc22 100644
--- a/src/gprs/gb_proxy.c
+++ b/src/gprs/gb_proxy.c
@@ -70,7 +70,7 @@
 };
 
 static const struct rate_ctr_group_desc global_ctrg_desc = {
-	.group_name_prefix = "gbproxy.global",
+	.group_name_prefix = "gbproxy:global",
 	.group_description = "GBProxy Global Statistics",
 	.num_ctr = ARRAY_SIZE(global_ctr_description),
 	.ctr_desc = global_ctr_description,