KafkaSubscriber: Fix ClassCanBeStatic warning

When all error prone warnings are enabled the ClassCanBeStatic bug
pattern is reported:

Configuration.java:343: error: [ClassCanBeStatic] Inner class is \
non-static but does not reference enclosing class
  public class KafkaSubscriber extends Properties {
         ^
    (see https://errorprone.info/bugpattern/ClassCanBeStatic)

Change-Id: I300f88ea3655a199320f442d2fe10775077d30cd
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/Configuration.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/Configuration.java
index 72fec47..3ef0f7d 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/Configuration.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/Configuration.java
@@ -340,7 +340,7 @@
     }
   }
 
-  public class KafkaSubscriber extends Properties {
+  public static class KafkaSubscriber extends Properties {
     private static final long serialVersionUID = 1L;
 
     static final String KAFKA_SUBSCRIBER_SUBSECTION = "subscriber";