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

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@39428 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/univ/checkbox.cpp

@@ -25,9 +25,10 @@
 
 #if wxUSE_CHECKBOX
 
+#include "wx/checkbox.h"
+
 #ifndef WX_PRECOMP
     #include "wx/dcclient.h"
-    #include "wx/checkbox.h"
     #include "wx/validate.h"
 
     #include "wx/button.h" // for wxACTION_BUTTON_XXX