xserver-xorg: fix build when Xtst isn't available

The optional DMX support (Distributed Multihead X) needs Xtst, which
may or may not be available in the buildroot config. DMX is unlikely
to be needed for a BR system, so simply disable it rather than
forcibly requiring Xtst.

Fixes http://autobuild.buildroot.org/results/df990ded6472de62132e3a089634fafe0e029107/



Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>