Build system: MT_LIBS includes XLIBS

Build system: MT_LIBS includes XLIBS

The MT_LIBS library list already includes XLIBS, so there's no need
to specify both on a link line.

Change-Id: I8594b1b6e1a16af741b40822cbce49e846b26f49
Reviewed-on: http://gerrit.openafs.org/8904
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>