AA: generic: fix UBIFS build error if XZ compression is disabled

Backport of r34959.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@34978 3c298f89-4303-0410-b956-a3cf2f4a3e73