Remove build-only include from public header

The dix-config.h file is not installed, but dix.h is. The include makes the
compilation of external drivers fail (for example the libinput driver).

The Xserver compilation also works without the include, so just remove it.

Signed-off-by: Roman Gilg <subdiff@gmail.com>
5 jobs for compile-fix in 7 minutes and 24 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #12590517

00:00:16

 
  Build And Test
passed autotools #12590518

00:07:08

passed meson #12590519

00:02:32

passed meson-noglamor #12590520

00:02:44

passed mingw-cross-build #12590521

00:01:10