opus: disable docs option in meson fallback

As the docs are not necessary, no need to keep it
enabled.

Part-of: <gstreamer/gst-plugins-base!943>

ext/opus/meson.build

@@ -7,7 +7,7 @@ opus_sources = [
 ]
 
 opus_dep = dependency('opus', version: '>= 0.9.4', required: get_option('opus'),
-                      fallback: ['opus', 'opus_dep'])
+                      fallback: ['opus', 'opus_dep'], default_options: ['docs=disabled'])
 
 if opus_dep.found()
   gstopus = library('gstopus',