[AMBARI-24865] Build error at Findbugs with Maven 3.6.

[adoroszlai]

What changes were proposed in this pull request?

Same as #2581, for branch-2.6, because now Jenkins uses Maven 3.6, too:

Apache Maven 3.6.0 (97c98ec64a1fdfee7767ce5ffb20918da4f719f3; 2018-10-24T18:41:47Z)

How was this patch tested?

Build with Maven 3.6:

$ mvn -version
Apache Maven 3.6.0 (97c98ec64a1fdfee7767ce5ffb20918da4f719f3; 2018-10-24T20:41:47+02:00)
...
$ mvn -am -pl ambari-server -DskipTests clean verify
...
[INFO] Ambari Server 2.6.2.0.0 ............................ SUCCESS [04:41 min]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/Ambari-Github-PullRequest-Builder/4895/
Test PASSed.