Replace SDL_strdup() by strdup()

The functions SDL_malloc(), SDL_free() and SDL_strdup() were used only
because strdup() was not available everywhere.

Now that it is available, use the native version of these functions.

app/src/device_msg.h

@@ -19,7 +19,7 @@ struct device_msg {
     enum device_msg_type type;
     union {
         struct {
-            char *text; // owned, to be freed by SDL_free()
+            char *text; // owned, to be freed by free()
         } clipboard;
     };
 };