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.c

@@ -121,7 +121,7 @@ file_handler_destroy(struct file_handler *file_handler) {
     SDL_DestroyCond(file_handler->event_cond);
     SDL_DestroyMutex(file_handler->mutex);
     request_queue_destroy(&file_handler->queue);
-    SDL_free((void *) file_handler->serial);
+    SDL_free(file_handler->serial);
 }
 
 static process_t

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;