util-core: Fix Scaladoc Generation
util-core: Fix Scaladoc Generation

Summary: Problem / Solution

Scaldoc generation is failing for `util-core` because of `Monitors.java`. It is
unable to find the Scala companion object for `Monitor`, `Monitor$`.
This change excludes `Monitors.java` from the sources used for scaladoc
generation, so that it may proceed successfully.

JIRA Issues: CSL-5034

Differential Revision: https://phabricator.twitter.biz/D79748

twitter/util,twitter/util,twitter/util