getcwd: fix compilation on mingw64

getcwd: fix compilation on mingw64

Mingw 64 added <direct.h> as a place that declared getcwd.  It is
a non-standard header, but since we already include it in the
replacement for <sys/stat.h>, we must also include it in the
replacement for <unistd.h> to avoid compilation problems.

* lib/unistd.in.h (includes) [mingw]: Include <direct.h> for
getcwd.
Reported by Marc-André Lureau.

Signed-off-by: Eric Blake <eblake@redhat.com>