Each .cpp/.mm file includes corresponding header before any other...

... except Audacity.h

This forces us to make each header contain all forward declarations or nested
headers that it requires, rather than depend on context.

src/import/ImportGStreamer.cpp

@@ -25,11 +25,12 @@ Licensed under the GNU General Public License v2 or later
 
 #include "../Audacity.h"	// needed before GStreamer.h
 
+#if defined(USE_GSTREAMER)
+#include "ImportGStreamer.h"
+
 #include <wx/window.h>
 #include <wx/log.h>
 
-#if defined(USE_GSTREAMER)
-
 #include "../MemoryX.h"
 
 #define DESC _("GStreamer-compatible files")
@@ -54,7 +55,6 @@ Licensed under the GNU General Public License v2 or later
 #include "../WaveTrack.h"
 #include "Import.h"
 #include "ImportPlugin.h"
-#include "ImportGStreamer.h"
 
 extern "C"
 {