Make wxRescaleCoord() private functions

They're probably not that useful in application code, which should just
use wxDPIChangedEvent::Scale() instead, so move them to a private
header instead of making them part of the public API.

src/msw/button.cpp

@@ -42,6 +42,7 @@
 #include "wx/stockitem.h"
 #include "wx/msw/private/button.h"
 #include "wx/msw/private/dc.h"
+#include "wx/private/rescale.h"
 #include "wx/private/window.h"
 
 #if wxUSE_MARKUP