lmb: remove unused extern declaration

lmb.h includes an extern declaration of "struct lmb lmb;" which
is not used anywhere, so remove it.
Reviewed-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NSimon Goldschmidt <simon.k.r.goldschmidt@gmail.com>

include/lmb.h

@@ -28,8 +28,6 @@ struct lmb {
 	struct lmb_region reserved;
 };
 
-extern struct lmb lmb;
-
 extern void lmb_init(struct lmb *lmb);
 extern void lmb_init_and_reserve(struct lmb *lmb, phys_addr_t base,
 				 phys_size_t size, void *fdt_blob);