Platform: use const argv in Sdl2Application

src/Magnum/Platform/Sdl2Application.h

@@ -485,10 +485,10 @@ class Sdl2Application {
         /** @brief Application arguments */
         struct Arguments {
             /** @brief Constructor */
-            /*implicit*/ constexpr Arguments(int& argc, char** argv) noexcept: argc{argc}, argv{argv} {}
+            /*implicit*/ constexpr Arguments(int& argc, const char* const* argv) noexcept: argc{argc}, argv{argv} {}
 
             int& argc;      /**< @brief Argument count */
-            char** argv;    /**< @brief Argument values */
+            const char* const* const argv;    /**< @brief Argument values */
         };
 
         class Configuration;