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

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@39957 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/bitmap.cpp

@@ -18,10 +18,10 @@
     #include "wx/palette.h"
     #include "wx/icon.h"
     #include "wx/math.h"
+    #include "wx/image.h"
 #endif
 
 #include "wx/filefn.h"
-#include "wx/image.h"
 
 #include "wx/rawbmp.h"
     // need this to get gdk_image_new_bitmap()