Revert "Break cycle of FileDialog and FileDialogPrivate"...

... It didn't really break the cycle, because there were still indirect
inclusions via AudacityHeaders.h.

This reverts commit 4f78874978.

src/widgets/FileDialog/gtk/FileDialogPrivate.cpp

@@ -11,6 +11,8 @@
 
 #include <gtk/gtk.h>
 
+#include "../FileDialog.h"
+
 #include "wx/intl.h"
 #include "wx/msgdlg.h"
 

src/widgets/FileDialog/mac/FileDialogPrivate.h

@@ -14,6 +14,8 @@
 #ifndef _MAC_FILEDIALOG_H_
 #define _MAC_FILEDIALOG_H_
 
+#include "../FileDialog.h"
+
 class wxChoice;
 
 //-------------------------------------------------------------------------

src/widgets/FileDialog/mac/FileDialogPrivate.mm

@@ -24,6 +24,8 @@
 // For compilers that support precompilation, includes "wx.h".
 #include <wx/wxprec.h>
 
+#include "../FileDialog.h"
+
 #ifndef WX_PRECOMP
     #include <wx/msgdlg.h>
     #include <wx/app.h>

src/widgets/FileDialog/win/FileDialogPrivate.cpp

@@ -49,6 +49,8 @@
 #include <wx/modalhook.h>
 #include <wx/filectrl.h>
 
+#include "../FileDialog.h"
+
 #include <shlobj.h>
 
 // ----------------------------------------------------------------------------