mozjs_17.0.0.bb: Fix do_srpm error

The function do_sourceclean() will lead to do_srpm failed, so use
do_unpack instead.

Signed-off-by: Li Xin <lixin.fnst@cn.fujitsu.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>