FxMixer, FxMixerView: export class symbols

Due to the FxMixer improvements some plugins such as the FLP import
filter now directly access the FxMixer and its view. Therefore the
symbols of the FxMixer and FxMixerView classes need to be exported for
the win32 builds.

include/FxMixer.h

@@ -69,7 +69,7 @@ class FxChannel : public ThreadableJob
 
 
 
-class FxMixer : public JournallingObject, public Model
+class EXPORT FxMixer : public JournallingObject, public Model
 {
 public:
 	FxMixer();

include/FxMixerView.h

@@ -42,7 +42,7 @@
 class QButtonGroup;
 class FxLine;
 
-class FxMixerView : public QWidget, public ModelView,
+class EXPORT FxMixerView : public QWidget, public ModelView,
 					public SerializingObjectHook
 {
 	Q_OBJECT