Re-add warning suppression

The IDE doesn't think this suppression is needed, but javac has a
warning and we have -Werror enabled.

HitHashMapOfCollection.java:30: warning: [serial] non-transient instance field of a serializable class declared with a non-serializable type
  protected Statistics<K> statistics;
                          ^

Change-Id: I2d1c4d6ef8ff58b0ef6e3f2a9cb80003e4e7d3b8

src/main/java/com/googlesource/gerrit/plugins/task/statistics/HitHashMapOfCollection.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/task/statistics/HitHashMapOfCollection.java b/src/main/java/com/googlesource/gerrit/plugins/task/statistics/HitHashMapOfCollection.java
index 2b90d2c..a6f83ac 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/task/statistics/HitHashMapOfCollection.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/task/statistics/HitHashMapOfCollection.java
@@ -27,6 +27,7 @@
 
   private static final long serialVersionUID = 1;
 
+  @SuppressWarnings("serial")
   protected Statistics<K> statistics;
 
   public HitHashMapOfCollection() {}