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.

include/wx/event.h

@@ -3142,8 +3142,8 @@ public:
 
     // Scale the value by the ratio between new and old DPIs carried by this
     // event.
-    int ScaleX(int x) const { return wxRescaleCoord(x, m_newDPI.x, m_oldDPI.x); }
-    int ScaleY(int y) const { return wxRescaleCoord(y, m_newDPI.y, m_oldDPI.y); }
+    int ScaleX(int x) const;
+    int ScaleY(int y) const;
 
     wxSize Scale(wxSize sz) const { return wxSize(ScaleX(sz.x), ScaleY(sz.y)); }