Register GSM_BTS_TYPE_UNKNOWN in bsc_hack.c

The way the VTY configuration sytem works is that it first registers a
BTS of type GSM_BTS_TYPE_UNKNOWN and then sets the type correctly (after
encountering the type statement). This makes sure that registering a BTS
of type UNKNOWN succeeds.
diff --git a/openbsc/src/bsc_hack.c b/openbsc/src/bsc_hack.c
index e7db3c7..7755726 100644
--- a/openbsc/src/bsc_hack.c
+++ b/openbsc/src/bsc_hack.c
@@ -178,6 +178,7 @@
 	bsc_schedule_timer(&db_sync_timer, DB_SYNC_INTERVAL);
 }
 
+extern int bts_model_unknown_init(void);
 extern int bts_model_bs11_init(void);
 extern int bts_model_nanobts_init(void);
 
@@ -194,6 +195,7 @@
 	stderr_target = debug_target_create_stderr();
 	debug_add_target(stderr_target);
 
+	bts_model_unknown_init();
 	bts_model_bs11_init();
 	bts_model_nanobts_init();