package:grub2: fix build error on linux missing libzfs

configure enables libzfs support on default.
This will break the build, on systems without libzfs.

Signed-off-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>

Backport of r41935

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@41975 3c298f89-4303-0410-b956-a3cf2f4a3e73