libxfs-apply: don't add duplicate headers

libxfs-apply: don't add duplicate headers

When we're backporting patches from libxfs, don't add a S-o-b header if
there's already one at the end of the headers of the patch being ported.

That way, we avoid things like:
 Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
 Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>