libxservertest needs -lpthread from glxapi.c's pthread_once() call. Usually
this would be pulled in by the XORG_LIBS but not when building without Xorg.
This commit has no visible effect on the current tree, preparation for test
cleanups.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>