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.

include/dix.h

@@ -54,7 +54,6 @@ SOFTWARE.
 #include "cursor.h"
 #include "geext.h"
 #include "events.h"
-#include <dix-config.h>
 #include <X11/extensions/XI.h>
 
 #define EARLIER -1