dix: don't install selection.h anymore

This file isn't included by any driver - not even indirectly, and hard
to imagine any driver ever needs it, so no need to keep it installed.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

include/meson.build

@@ -487,7 +487,6 @@ if build_xorg
             'rgb.h',
             'screenint.h',
             'scrnintstr.h',
-            'selection.h',
             'servermd.h',
             'validate.h',
             'displaymode.h',