gbproxy: Reset IMSI acquisition within gbproxy_unregister_link_info

Currently then link_info is not cleaned up completely, when
gbproxy_unregister_link_info is called.

This patch adds a function gbproxy_reset_link that must be defined
externally. This is done in gb_proxy.c, where it resets the IMSI
acquisition.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/include/openbsc/gb_proxy.h b/openbsc/include/openbsc/gb_proxy.h
index 1d1812f..c1882f3 100644
--- a/openbsc/include/openbsc/gb_proxy.h
+++ b/openbsc/include/openbsc/gb_proxy.h
@@ -224,6 +224,7 @@
 uint32_t gbproxy_make_sgsn_tlli(
 	struct gbproxy_peer *peer, struct gbproxy_link_info *link_info,
 	uint32_t bss_tlli);
+void gbproxy_reset_link(struct gbproxy_link_info *link_info);
 int gbproxy_check_imsi(
 	struct gbproxy_peer *peer, const uint8_t *imsi, size_t imsi_len);