License check for jdiff output files should be ignored

The following commits added jdiff output for Hadoop 2.8.0 but ASF license header is missing.

• https://github.com/apache/hadoop/commit/6df029db36412b7219b64313dcbe1874dc1c8b0c
• https://github.com/apache/hadoop/commit/d174c06b01e1f743d3111b9b760a9824d8106b86

hadoop-mapreduce-project module does not have a setting to ignore the jdiff output files, so the license check fails.