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: <!1905>
83 jobs for !1905 with xxf86vm in 16 minutes and 10 seconds (queued for 8 seconds)
merge request