core/frontend/emu_window: Make ClipToTouchScreen a const member function

This member function doesn't modify instance state, so it can have the
const specifier applied to it.

src/core/frontend/emu_window.h

@@ -164,5 +164,5 @@ private:
     /**
      * Clip the provided coordinates to be inside the touchscreen area.
      */
-    std::tuple<unsigned, unsigned> ClipToTouchScreen(unsigned new_x, unsigned new_y);
+    std::tuple<unsigned, unsigned> ClipToTouchScreen(unsigned new_x, unsigned new_y) const;
 };