Avoid ignoring Gradle wrapper jar file

This is added because .jar files are usually ignored (see Java.gitignore)

Gradle.gitignore

@@ -3,3 +3,6 @@ build/
 
 # Ignore Gradle GUI config
 gradle-app.setting
+
+# Avoid ignoring Gradle wrapper jar file (.jar files are usually ignored)
+!gradle-wrapper.jar