Fix lack of repeat-inclusion guard in win32 substitute sys/socket.h.

Fix lack of repeat-inclusion guard in win32 substitute sys/socket.h.
I think this explains the 'implicit declaration of function gai_strerror'
warnings visible in the current buildfarm report from snake: if
sys/socket.h is included again after getaddrinfo.h, the file would
merrily undefine the gai_strerror macro.