package/mesa3d: Add optional dependency xlib_libXxf86vm

xlib_libXxf86vm is an optional dependency for the DRI drivers,
quote from mesa3d/configure.ac:

    # add xf86vidmode if available
    PKG_CHECK_MODULES([XF86VIDMODE], [xxf86vm], HAVE_XF86VIDMODE=yes, HAVE_XF86VIDMODE=no)
    if test "$HAVE_XF86VIDMODE" = yes ; then
        dri_modules="$dri_modules xxf86vm"
    fi

Add the package as an optional dependency to have reproducable builds.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>