Volume control for QAudioInput windows backend.

Change-Id: I1245a38c68254fc48373a856dc44e8ad8142636e
Reviewed-by: Ling Hu <ling.hu@nokia.com>

src/multimedia/audio/qaudioinput_win32_p.h

@@ -114,6 +114,8 @@ public:
     qint64 elapsedUSecs() const;
     QAudio::Error error() const;
     QAudio::State state() const;
+    void setVolume(qreal volume);
+    qreal volume() const;
 
     QIODevice* audioSource;
     QAudioFormat settings;
@@ -150,6 +152,11 @@ private:
     bool open();
     void close();
 
+    void initMixer();
+    void closeMixer();
+    UINT mixerID;
+    MIXERLINECONTROLS mixerLineControls;
+
 private slots:
     void feedback();
     bool deviceReady();