gbproxy: Fix rate counter group leak in peer_free()

This also frees the counter group pointed to by peer->ctrg when the
peer gets freed.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gb_proxy.c b/openbsc/src/gprs/gb_proxy.c
index dea6e3f..16a77b4 100644
--- a/openbsc/src/gprs/gb_proxy.c
+++ b/openbsc/src/gprs/gb_proxy.c
@@ -207,6 +207,7 @@
 static void peer_free(struct gbprox_peer *peer) __attribute__((__unused__));
 static void peer_free(struct gbprox_peer *peer)
 {
+	rate_ctr_group_free(peer->ctrg);
 	llist_del(&peer->list);
 	talloc_free(peer);
 }