QtMultimedia: Fix compiler warnings.

Change-Id: I5783b1f6dce645fb473e7d3da0911dbb40e3951b
Reviewed-by: Jonathan Liu <net147@gmail.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/multimedia/audio/qaudiodeviceinfo_win32_p.cpp

@@ -309,7 +309,7 @@ void QAudioDeviceInfoInternal::updateLists()
 {
     // redo all lists based on current settings
     bool match = false;
-    DWORD fmt = NULL;
+    DWORD fmt = 0;
 
     if(mode == QAudio::AudioOutput) {
         WAVEOUTCAPS woc;