python: default the destructor also in the SDL2 application.

src/python/magnum/platform/sdl2.cpp

@@ -64,7 +64,7 @@ void sdl2(py::module_& m) {
 
         /* The base doesn't have a virtual destructor because in C++ it's never
            deleted through a pointer to the base. Here we need it, though. */
-        virtual ~PublicizedApplication() {}
+        virtual ~PublicizedApplication() = default;
     };
 
     struct PyApplication: PublicizedApplication {