Fix: resolve grpServReg compilation error caused by checkstyle

Change-Id: I9fa2eef759bac1fb042998d9e715b73123aabbeb
diff --git a/incubator/protobuf/registry/src/main/java/org/onosproject/protobuf/registry/GrpcServiceRegistryImpl.java b/incubator/protobuf/registry/src/main/java/org/onosproject/protobuf/registry/GrpcServiceRegistryImpl.java
index 2304f0e..ada1253 100644
--- a/incubator/protobuf/registry/src/main/java/org/onosproject/protobuf/registry/GrpcServiceRegistryImpl.java
+++ b/incubator/protobuf/registry/src/main/java/org/onosproject/protobuf/registry/GrpcServiceRegistryImpl.java
@@ -35,6 +35,7 @@
 import java.util.Dictionary;
 import java.util.Map;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import static org.onlab.util.Tools.get;
 
@@ -52,6 +53,7 @@
 
     private static final int DEFAULT_SERVER_PORT = 64000;
     private static final int DEFAULT_SHUTDOWN_TIME = 1;
+    private static final AtomicBoolean SERVICES_MODIFIED_SINCE_START = new AtomicBoolean(false);
 
     private static final String PORT_PROPERTY_NAME = "listeningPort";