Make wxWindow::MSWGetThemeColour() const.

There doesn't seem to be any reason for this method to not be const.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63269 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/window.h

@@ -428,7 +428,7 @@ public:
                                int themePart,
                                int themeState,
                                MSWThemeColour themeColour,
-                               wxSystemColour fallback);
+                               wxSystemColour fallback) const;
 
     // gives the parent the possibility to draw its children background, e.g.
     // this is used by wxNotebook to do it using DrawThemeBackground()