add copyright notice

app/src/main/java/com/m2049r/xmrwallet/layout/DiffCallback.java

@@ -1,3 +1,19 @@
+/*
+ * Copyright (c) 2021 yorha-0x
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 package com.m2049r.xmrwallet.layout;
 
 import androidx.recyclerview.widget.DiffUtil;