Update hdfs makefile to use .c instead of .cc suffix.

From @mppf:

I was getting build problems that 'make clobber' solved for me. I believe the
patch below addresses the issue, since the HDFS support files are now C.
Merge pull request #222 from thomasvandoren/fix-hdfs-makefile

Update hdfs makefile to use .c instead of .cc suffix.

From @mppf:

I was getting build problems that 'make clobber' solved for me. I believe the
patch below addresses the issue, since the HDFS support files are now C.