Stop passing WNDPROC argument to wxCheckWindowWndProc()

This parameter wasn't used anyhow and casting wxWndProc or m_oldWndProc
to WXFARPROC just resulted in gcc8 -Wcast-function-type warnings.

Get rid of them by not passing the window proc to this function at all.

include/wx/msw/private.h

@@ -936,7 +936,10 @@ extern WXDLLIMPEXP_CORE int wxGetWindowId(WXHWND hWnd);
 
 // check if hWnd's WNDPROC is wndProc. Return true if yes, false if they are
 // different
-extern WXDLLIMPEXP_CORE bool wxCheckWindowWndProc(WXHWND hWnd, WXFARPROC wndProc);
+//
+// wndProc parameter is unused and only kept for compatibility
+extern WXDLLIMPEXP_CORE
+bool wxCheckWindowWndProc(WXHWND hWnd, WXFARPROC wndProc = NULL);
 
 // Does this window style specify any border?
 inline bool wxStyleHasBorder(long style)