fix compile error

Author: hailshao

log4j-core/src/main/java/org/apache/logging/log4j/core/impl/ThrowableProxyHelper.java

@@ -24,6 +24,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import org.apache.logging.log4j.core.util.Loader;
 import org.apache.logging.log4j.status.StatusLogger;
 import org.apache.logging.log4j.util.LoaderUtil;