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/src/gprs/gb_proxy_tlli.c b/openbsc/src/gprs/gb_proxy_tlli.c
index fdb1509..fae05ed 100644
--- a/openbsc/src/gprs/gb_proxy_tlli.c
+++ b/openbsc/src/gprs/gb_proxy_tlli.c
@@ -367,6 +367,8 @@
 
 	link_info->is_deregistered = 1;
 
+	gbproxy_reset_link(link_info);
+
 	return;
 }