Add Gradle wrapper SHA256 sum

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,23 @@
+#
+# Léon - The URL Cleaner
+# Copyright (C) 2021 Sven Jacobs
+#
+# 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 <http://www.gnu.org/licenses/>.
+#
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
+distributionSha256Sum=13bf8d3cf8eeeb5770d19741a59bde9bd966dd78d17f1bbad787a05ef19d1c2d
 distributionUrl=https\://services.gradle.org/distributions/gradle-7.0.2-all.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists