Only include <unistd.h> under Unix in wxGTK code.

Also remove the inclusions of some Unix-specific headers that were not needed
at all.

Closes #14048.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70704 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/filedlg.cpp

@@ -22,7 +22,9 @@
 #include <gtk/gtk.h>
 #include "wx/gtk/private.h"
 
+#ifdef __UNIX__
 #include <unistd.h> // chdir
+#endif
 
 #include "wx/filename.h" // wxFilename
 #include "wx/tokenzr.h" // wxStringTokenizer