Include wx/app.h according to precompiled headers of wx/wx.h (with other minor cleaning).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38945 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/os2/dnd.cpp

@@ -14,11 +14,15 @@
 
 #if wxUSE_DRAG_AND_DROP
 
-#include "wx/window.h"
-#include "wx/app.h"
-#include "wx/gdicmn.h"
 #include "wx/dnd.h"
 
+#ifndef WX_PRECOMP
+    #include "wx/app.h"
+#endif
+
+#include "wx/window.h"
+#include "wx/gdicmn.h"
+
 #define INCL_PM
 #define INCL_DOS
 #include <os2.h>