lguest: remove unnecessary forward struct declaration

While fixing lg.h to drop the fwd declaration, I noticed
there's another one ;)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/lguest/lg.h

@@ -38,8 +38,6 @@ struct lguest_pages
 #define CHANGED_GDT_TLS		4 /* Actually a subset of CHANGED_GDT */
 #define CHANGED_ALL	        3
 
-struct lguest;
-
 struct lg_cpu {
 	unsigned int id;
 	struct lguest *lg;