scripts/extract-ikconfig: add xz compression support

Add support for kernels compressed with xz to the extract-ikconfig script.

Signed-off-by: Dick Streefland <dick@streefland.net>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>