Minimize extra includes to fix build

There's some kind of gnulib build error caused by include unistd.h and
sys/types.h as I was directed to do by stat(2).

Dropping down to including just sys/stat.h fixes this problem.

As far as I can see, POSIX says that this is fine to get the required
definitions for the stat function and structure:
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/sys/stat.h.html