include: unexport xserver_poll.h

It doesn't seem to have any exported functions, so doesn't need to be
exported to the public module API.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1352>

include/meson.build

@@ -503,7 +503,6 @@ if build_xorg
             'xkbstr.h',
             'xkbrules.h',
             'Xprintf.h',
-            'xserver_poll.h',
             'xserver-properties.h',
         ],
         install_dir: xorgsdkdir,