Fix clang style warnings in webrtc/modules/audio_coding/neteq

Mostly this consists of marking functions with override when
applicable, and moving function bodies from .h to .cc files.

BUG=163
R=henrik.lundin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44109004

Cr-Commit-Position: refs/heads/master@{#8960}

webrtc/modules/audio_coding/neteq/audio_classifier.h

@@ -37,7 +37,7 @@ class AudioClassifier {
   bool Analysis(const int16_t* input, int input_length, int channels);
 
   // Gets the current classification : true = music, false = speech.
-  virtual bool is_music() const { return is_music_; }
+  virtual bool is_music() const;
 
   // Gets the current music probability.
   float music_probability() const { return music_probability_; }