Fix PlayerBirthday exception

Signed-off-by: Jaida Wu <mlgmxyysd@meowcat.org>

src/main/java/emu/grasscutter/game/player/PlayerBirthday.java

 package emu.grasscutter.game.player;
 
+import dev.morphia.annotations.Entity;
 import emu.grasscutter.net.proto.BirthdayOuterClass.Birthday;
 
+@Entity
 public class PlayerBirthday {
     private int day;
     private int month;