xlib_libXt: add patch to fix cross-compilation issue

Fixes the following build failure:

 http://autobuild.buildroot.org/results/2815885d589d75259508bcce4f272b8dddb3be77/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>