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/xml/xml.cpp

@@ -22,6 +22,7 @@
 #ifndef WX_PRECOMP
     #include "wx/intl.h"
     #include "wx/log.h"
+    #include "wx/app.h"
 #endif
 
 #include "wx/wfstream.h"
@@ -32,7 +33,6 @@
 #include "expat.h" // from Expat
 
 // DLL options compatibility check:
-#include "wx/app.h"
 WX_CHECK_BUILD_OPTIONS("wxXML")