auxiliary/vl: Build vl_winsys_dri.c only when needed.

auxiliary/vl: Build vl_winsys_dri.c only when needed.

With commit c39dbfdd0f7(auxiliary/vl: bring back the VL code for the dri
targets) we did not fully consider users of dri-swrast alone. Thus we
ended up trying to compile the dri2 specific code on platform which lack
it - Cygwin for example.

Cc: "10.5" <mesa-stable@lists.freedesktop.org>
Reported-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>