[SPARK-3347] [yarn] Fix yarn-alpha compilation.

Missing import. Oops.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #2236 from vanzin/SPARK-3347 and squashes the following commits:

594fc39 [Marcelo Vanzin] [SPARK-3347] [yarn] Fix yarn-alpha compilation.