meson.build: xxf86vm is not needed for -Dglx-direct=false

It is only used in src/glx/glxcmds.c and when GLX_DIRECT_RENDERING is
defined.
So only depend on it if GLX direct rendering is actually enabled.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1905>
1 file changed