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
1 file changed