gethostname: fix build on mingw

gethostname: fix build on mingw

Since commit 4e86671a, gethostname failed to compile on mingw.
Gnulib's sys/socket.h includes winsock2.h, which then includes
unistd.h prior to declaring gethostname.  The fix is to ensure
that our replacement unistd.h does not declare any replacements
until we are sure that winsock2.h is completely included.

* lib/unistd.in.h (includes): Work around fact that mingw
<winsock2.h> re-includes <unistd.h>, by avoiding any
redeclarations if we are being included by <winsock2.h>.
Reported by Matthias Bolte.

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