meson: Link xvmc with libxv

Prior to xvmc 1.0.12 libxvmc incorrectly required libxv, but that was
fixed. This results in compilation failures for the gallium xvmc tracker
and tools. This patch fixes that by explicitly linking to libxv.

Fixes: 22a817af
       ("meson: build gallium xvmc state tracker")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1844


Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit e456a053)