KafkaProperties: Mark it as singleton

Change-Id: I416ad8b6a69061eecc66ce393bf098b7121cf7f2
diff --git a/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaProperties.java b/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaProperties.java
index 24d9438..04195a2 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaProperties.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaProperties.java
@@ -19,7 +19,9 @@
 import com.google.gerrit.server.config.PluginConfig;
 import com.google.gerrit.server.config.PluginConfigFactory;
 import com.google.inject.Inject;
+import com.google.inject.Singleton;
 
+@Singleton
 public class KafkaProperties extends java.util.Properties {
   private static final long serialVersionUID = 0L;