Remove build-only include from public header

Roman Gilg requested to merge romangg/xserver:compile-fix into master

Since 5c2592cb the dix-config.h is included in the public dix.h header.

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.

@DemiMarie @p12tic

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