mingw: ensure sockets are initialized before calling gethostname

mingw: ensure sockets are initialized before calling gethostname

If the Windows sockets subsystem has not been initialized yet then an
attempt to get the hostname returns an error and prints a warning to the
console. This solves this issue for msysGit as seen with 'git fetch'.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>