kvm tools: set "done" to true on SDL window close

Currently, SDL doesn't exit on window close because it stuck in
sdl__stop because "done" is not set to true.

Fix it by setting "done" to true on window close.

Signed-off-by: Lin Ming <minggr@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

ui/sdl.c

@@ -265,6 +265,7 @@ static void *sdl__thread(void *p)
 		return NULL;
 	}
 exit:
+	done = true;
 	kvm_cpu__reboot(fb->kvm);
 
 	return NULL;