Fix build against Gerrit 'stable-3.8'+ versions

Adjust to interface changes applied with [1]:
  RateLimitReachedSender.java:70: error: cannot find symbol
    add(RecipientType.TO, user.getAccountId());
    ^
  symbol:   method add(RecipientType,Id)
  location: class RateLimitReachedSender

[1] https://gerrit-review.googlesource.com/c/gerrit/+/364496

Change-Id: Ia7ec1d5be6d8efe3c6343418e157a98e0ad56c5a
1 file changed