libx11: backport _XEatDataWords API

If you build libx11-native then that has to be ABI-compatible with the
libX11 on the host or you'll have problems running qemu-native. Most
current distros are using libX11 1.6+. Thus, we need to backport the
_XEatDataWords API present in 1.6.

This only affects the dylan branch as dora+ has libx11 1.6+.

Fixes [YOCTO #5040].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>