Changed muxer listener to use "default"

Changed muxer listener to use "default" instead of {}
when defining its constructor and destructor in its
header file.

Change-Id: I3c80e808aa624293c3d7720ac6a47b470a663312

packager/media/event/muxer_listener.h

@@ -53,7 +53,7 @@ class MuxerListener {
     std::vector<Range> subsegment_ranges;
   };
 
-  virtual ~MuxerListener() {};
+  virtual ~MuxerListener() = default;
 
   /// Called when the media's encryption information is ready.
   /// OnEncryptionInfoReady with @a initial_encryption_info being true should be
@@ -134,7 +134,7 @@ class MuxerListener {
   virtual void OnCueEvent(uint64_t timestamp, const std::string& cue_data) = 0;
 
  protected:
-  MuxerListener() {};
+  MuxerListener() = default;
 };
 
 }  // namespace media