sgsn: Add sgsn_ggsn_ctx_free function

This function will be needed for testing, since the leak check would
fail if the GGSN context are not cleaned up after use.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index 54fe15c..85f3381 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -352,6 +352,12 @@
 	return ggc;
 }
 
+void sgsn_ggsn_ctx_free(struct sgsn_ggsn_ctx *ggc)
+{
+	llist_del(&ggc->list);
+	talloc_free(ggc);
+}
+
 struct sgsn_ggsn_ctx *sgsn_ggsn_ctx_by_id(uint32_t id)
 {
 	struct sgsn_ggsn_ctx *ggc;