Fix a potential threadsafe issue

Lazily setting @mutex could lead to have two mutexes with a sample code
like:

    [MyListener.new, MailListener.new].each do |listener|
      Thread.new {
        Wisper::GlobalListeners.add_listener(listener)
      }
    end

This example is cumbersome, but well, it shows the idea. Even with a
singleton, if `add_listener` is called a first time by two different
threads, this could lead to thread safety issues for ruby implementation
without a gvl.

Signed-off-by: chatgris <jboyer@af83.com>

lib/wisper/global_listeners.rb

@@ -3,9 +3,12 @@
 module Wisper
   class GlobalListeners
     include Singleton
+    attr_reader :mutex
+    private :mutex
 
     def initialize
       @listeners = Set.new
+      @mutex     = Mutex.new
     end
 
     def add_listener(listener, options = {})
@@ -35,10 +38,6 @@ def self.clear
 
     private
 
-    def mutex
-      @mutex ||= Mutex.new
-    end
-
     def with_mutex
       mutex.synchronize { yield }
     end