signal.h: Don't include gsm_subscriber but rather declare it

openbsc/include/openbsc/signal.h

@@ -26,7 +26,6 @@
 #include <errno.h>
 
 #include <openbsc/gsm_data.h>
-#include <openbsc/gsm_subscriber.h>
 
 #include <osmocore/signal.h>
 
@@ -118,6 +117,8 @@ enum signal_global {
 	S_GLOBAL_SHUTDOWN,
 };
 
+struct gsm_subscriber;
+
 struct paging_signal_data {
 	struct gsm_subscriber *subscr;
 	struct gsm_bts *bts;