include: unexport XIstubs.h

The functions declared here aren't used by any driver, so no need to keep
them in the public driver API. Since the whole file isn't included by anybody
outside the xserver tree itself, it doesn't need to be installed at all,
so making it internal and move it to Xi directory.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1325>

include/meson.build

@@ -439,7 +439,6 @@ endif
 if build_xorg
     install_data(
         [
-            'XIstubs.h',
             'Xprintf.h',
             'callback.h',
             'client.h',