meson: Try finding libxvmcw via pkg-config before using find_library

This fixes cross compiling issues, because pkg-config is less likely to
get the wrong libs.

v2: - Fix typo in comment

Fixes: 22a817af
       ("meson: build gallium xvmc state tracker")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/939


Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 8c5c21d7)