ns/stats: Fix class_id of "NSVC Peer Statistics" rate_ctr group

Currently the class_id is not set which effectively puts these groups
into the 'subscriber' class.

This commit adds the missing initialisation value.

Sponsored-by: On-Waves ehf
diff --git a/src/gb/gprs_ns.c b/src/gb/gprs_ns.c
index 2b189cd..d0be754 100644
--- a/src/gb/gprs_ns.c
+++ b/src/gb/gprs_ns.c
@@ -130,6 +130,7 @@
 	.group_description = "NSVC Peer Statistics",
 	.num_ctr = ARRAY_SIZE(nsvc_ctr_description),
 	.ctr_desc = nsvc_ctr_description,
+	.class_id = OSMO_STATS_CLASS_PEER,
 };
 
 enum ns_stat {