include: dont install glx_extinit.h

This header isn't included by any (known) driver/module and doesn't export
any functions, thus no need to install it anymore.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1361>
(cherry picked from commit c66eaf431c)

include/meson.build

@@ -474,7 +474,6 @@ if build_xorg
             'gc.h',
             'gcstruct.h',
             'globals.h',
-            'glx_extinit.h',
             'glxvndabi.h',
             'input.h',
             'inputstr.h',