out: fix module inheritance from Fluent:: to Fluent::Plugin for v0.14

ref. http://docs.fluentd.org/v0.14/articles/plugin-update-from-v12#buffered-output-plugins

lib/fluent/plugin/out_groonga.rb

@@ -22,6 +22,7 @@
 require "groonga/client"
 
 module Fluent
+  module Plugin
   class GroongaOutput < BufferedOutput
     Plugin.register_output("groonga", self)
 
@@ -759,4 +760,5 @@ def read_output(context)
       end
     end
   end
+  end
 end