nat: Close all endpoints used by a BSC when the BSC is gone

Cleanup all endpoints that belonged to a given BSC. This is
one part of the cleanup, the other is to bring down the SCCP
link properly.
diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index 13bdc71..6de32eb 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -205,6 +205,7 @@
 struct msgb *bsc_mgcp_rewrite(char *input, int length, const char *ip, int port);
 void bsc_mgcp_forward(struct bsc_connection *bsc, struct msgb *msg);
 
+void bsc_mgcp_clear_endpoints_for(struct bsc_connection *bsc);
 int bsc_mgcp_parse_response(const char *str, int *code, char transaction[60]);
 int bsc_mgcp_extract_ci(const char *resp);
 int bsc_mgcp_extract_port(const char *resp);