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>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 85fe728..c9b17f0 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -858,6 +858,7 @@
 
 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