USB: gadget-storage needs BLOCK

With CONFIG_BLOCK=n, this build error happens:
WARNING: "bdev_read_only" [drivers/usb/gadget/g_file_storage.ko] undefined!
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/Kconfig

@@ -370,6 +370,7 @@ config USB_GADGETFS
 
 config USB_FILE_STORAGE
 	tristate "File-backed Storage Gadget"
+	depends on BLOCK
 	help
 	  The File-backed Storage Gadget acts as a USB Mass Storage
 	  disk drive.  As its storage repository it can use a regular