MNCC: use explicit struct type in gsm_call

There's no need to use 'void *' because we have forward declaration for
'struct gsm_network' in the very same header.

Change-Id: I5078ffcf2706adaca1b5df107f8b6a44062ca28c

include/osmocom/msc/mncc.h

@@ -38,7 +38,7 @@ struct gsm_call {
 	struct llist_head entry;
 
 	/* network handle */
-	void *net;
+	struct gsm_network *net;
 
 	/* the 'local' transaction */
 	uint32_t callref;