GPRS/BSSGP: introduce bssgp_bvc_ctx_free()

So far we had a function to allocate a new bssgp_bvc_ctx, but not
the opposite one. Let's finally introduce it, so it will be used
at least in OsmoPCU.

Please note that the new symbol has 'bssgp_' prefix, not 'btsctx_'.

Change-Id: Ia78979379dbdccd6e4628c16f00d0c06d9212172
diff --git a/src/gb/gprs_bssgp.c b/src/gb/gprs_bssgp.c
index b695c28..896f1c5 100644
--- a/src/gb/gprs_bssgp.c
+++ b/src/gb/gprs_bssgp.c
@@ -141,6 +141,15 @@
 	return ctx;
 }
 
+void bssgp_bvc_ctx_free(struct bssgp_bvc_ctx *ctx)
+{
+	if (!ctx)
+		return;
+	rate_ctr_group_free(ctx->ctrg);
+	llist_del(&ctx->list);
+	talloc_free(ctx);
+}
+
 /* Chapter 10.4.5: Flow Control BVC ACK */
 static int bssgp_tx_fc_bvc_ack(uint16_t nsei, uint8_t tag, uint16_t ns_bvci)
 {