Add a volume (gain) property to QAudioInput.

Only implemented for PulseAudio so far, but the API does explain that
it's optional.

Change-Id: I4543a1c81d810fe92bb08f1ed13f3a3534a371e4
Reviewed-by: Ling Hu <ling.hu@nokia.com>

src/multimedia/audio/qaudioinput.h

@@ -91,6 +91,9 @@ public:
     void setNotifyInterval(int milliSeconds);
     int notifyInterval() const;
 
+    void setVolume(qreal volume);
+    qreal volume() const;
+
     qint64 processedUSecs() const;
     qint64 elapsedUSecs() const;