cmd: ubifs: Add a dependency on CMD_UBI

CMD_UBIFS can't compile without CMD_UBI enabled. Make sure we can't end up
in that case.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Heiko Schocher <hs@denx.de>

cmd/Kconfig

@@ -858,6 +858,7 @@ config CMD_UBI
 
 config CMD_UBIFS
 	tristate "Enable UBIFS - Unsorted block images filesystem commands"
+	depends on CMD_UBI
 	select CRC32
 	select RBTREE if ARCH_SUNXI
 	select LZO if ARCH_SUNXI