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/msw/combobox.cpp

@@ -26,15 +26,16 @@
 
 #if wxUSE_COMBOBOX
 
+#include "wx/combobox.h"
+
 #ifndef WX_PRECOMP
     #include "wx/settings.h"
     #include "wx/log.h"
     // for wxEVT_COMMAND_TEXT_ENTER
     #include "wx/textctrl.h"
+    #include "wx/app.h"
 #endif
 
-#include "wx/app.h"
-#include "wx/combobox.h"
 #include "wx/brush.h"
 #include "wx/clipbrd.h"
 #include "wx/msw/private.h"