image.h: add forward declaration of struct fdt_region

This header needs to know 'fdt_region' is a struct for the
fit_region_make_list() prototype.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/image.h

@@ -21,6 +21,7 @@
 
 /* Define this to avoid #ifdefs later on */
 struct lmb;
+struct fdt_region;
 
 #ifdef USE_HOSTCC
 #include <sys/types.h>