Allow for xz'd symvers file
    
    The Fedora/ARK kernel is moving to removing gzip as a dependency and
    replacing it with xz.
    
    Use xz instead of gz as an extension for the symvers file.
    
    Signed-off-by: Prarit Bhargava <prarit@redhat.com>
    (cherry picked from commit 78d64adfbe3cd089df8e41166c3688f026acdad1)
    [rharwood: bump spec]
    Signed-off-by: Robbie Harwood <rharwood@redhat.com>