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: 22a817af8a89eb3c762fc3e07b443a3ce37d7416
       ("meson: build gallium xvmc state tracker")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1844
Reviewed-by: Adam Jackson <ajax@redhat.com>
diff --git a/meson.build b/meson.build
index da4a075..aea5c2e 100644
--- a/meson.build
+++ b/meson.build
@@ -516,10 +516,12 @@
   endif
 endif
 dep_xvmc = null_dep
+dep_xv = null_dep
 with_gallium_xvmc = false
 if _xvmc != 'false'
   dep_xvmc = dependency('xvmc', version : '>= 1.0.6', required : _xvmc == 'true')
-  with_gallium_xvmc = dep_xvmc.found()
+  dep_xv = dependency('xv', required : _xvmc == 'true')
+  with_gallium_xvmc = dep_xvmc.found() and dep_xv.found()
 endif
 
 xvmc_drivers_path = get_option('xvmc-libs-path')