Make owned serial a pointer-to-non-const

The file handler owns the serial, so it needs to free it. Therefore, it
should not be a pointer-to-const.

app/src/file_handler.h

@@ -21,7 +21,7 @@ struct request_queue {
 };
 
 struct file_handler {
-    const char *serial;
+    char *serial;
     SDL_Thread *thread;
     SDL_mutex *mutex;
     SDL_cond *event_cond;