Address warning for log4j when compiling.

ragconnect.base/build.gradle

@@ -67,6 +67,9 @@ idea.module.generatedSourceDirs += genSrc
 jar {
     manifest {
         attributes "Main-Class": 'org.jastadd.ragconnect.compiler.Compiler'
+
+        // Log4J + Java 11 compatibility, see https://stackoverflow.com/q/53049346/2493208
+        attributes "Multi-Release": true
     }
 
     from {