cbfs: Fix corner case and compilation with recdent gcc

Accept the header to touch the jump address at 0xfffffff0.

Fix compilation for 64-bit EFI with recent GCC.

grub-core/fs/cbfs.c

@@ -352,7 +352,7 @@ init_cbfsdisk (void)
      reading ROMs through controller directly.
    */
   if (ptr < 0xff000000
-      || 0xffffffff - ptr < sizeof (*head) + 0x10
+      || 0xffffffff - ptr < (grub_uint32_t) sizeof (*head) + 0xf
       || !validate_head (head))
     return;