meson: Fix auto option for xvmc

This fixes the same problem as the previous patch did for vdpau, but for
xvmc.

Fixes: 724916c8a84b5bba8f880f17da936585d52c97b6
       ("meson: dedup gallium-xvmc logic")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
1 file changed