Add a declaration to QIODevice

It isn't getting #included nor forward-declared at this point. That was
only an indirect inclusion.

qaudiosystem.h:78:24: error: ‘QIODevice’ has not been declared

Change-Id: I5b9121b693bb06414638e8a4664115597da7466f
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

src/multimedia/audio/qaudiosystem.h

@@ -43,6 +43,8 @@
 
 QT_BEGIN_NAMESPACE
 
+class QIODevice;
+
 // Required for QDoc workaround
 class QString;