include: Add proper predeclaration for struct in mncc.h

We use them, so declare them. (avoid including all gsm_data.h)

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

openbsc/include/openbsc/mncc.h

@@ -28,6 +28,9 @@
 #include <osmocore/linuxlist.h>
 #include <osmocore/mncc.h>
 
+struct gsm_network;
+
+
 /* One end of a call */
 struct gsm_call {
 	struct llist_head entry;