meson: check for x11 when building the xv option

Obvious oversight in the original PR for the meson build. The xv option
requires x11 to function. Check for this by using the require method.

meson.build

@@ -1088,7 +1088,11 @@ if x11['use']
                      'video/out/x11_common.c')
 endif
 
-xv = dependency('xv', required: get_option('xv'))
+xv_opt = get_option('xv').require(
+    x11['use'],
+    error_message: 'x11 could not be found!',
+)
+xv = dependency('xv', required: xv_opt)
 if xv.found()
     dependencies += xv
     features += 'xv'