Fix licensing stuff

proxy/src/main/java/com/velocitypowered/proxy/event/EventTypeTracker.java

@@ -1,10 +1,25 @@
+/*
+ * Copyright (C) 2018 Velocity Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
 package com.velocitypowered.proxy.event;
 
 import com.google.common.collect.ArrayListMultimap;
-import com.google.common.collect.HashMultimap;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ListMultimap;
-import com.google.common.collect.Multimap;
 import com.google.common.reflect.TypeToken;
 import java.util.List;
 import java.util.stream.Collectors;