fix: 'm_state' was not declared in this scope

Error occurred when building with DEBUG_PLAYBIN defined.
It should be m_currentState from context.

Change-Id: Iad59966ca19c9d1b589fc8641599494436a0a5f3
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Jing Bai <doris.bai@gmail.com>

src/plugins/gstreamer/mediaplayer/qgstreamerplayercontrol.cpp

@@ -630,7 +630,7 @@ void QGstreamerPlayerControl::popAndNotifyState()
     if (m_stateStack.isEmpty()) {
         if (m_currentState != oldState) {
 #ifdef DEBUG_PLAYBIN
-            qDebug() << "State changed:" << m_state;
+            qDebug() << "State changed:" << m_currentState;
 #endif
             emit stateChanged(m_currentState);
         }