Add stdio.h for dirutils_test.cc

Change-Id: I62f322a4ecfcd46f0475bbe869c179b4d8976e3c
Reviewed-on: http://review.couchbase.org/31578
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>