fix gzip compress then it not supports rsyncable option

dracut fail to create initramfs when compressor gzip and
it not have rsyncable option

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
Merge branch 'master' of github.com:haraldh/dracut
Merge pull request #30 from vtolstov/master

fix gzip compress then it not supports rsyncable option