curl_addrinfo.c: Allow Unix Domain Sockets to compile under Windows

Windows 10.0.17061 SDK introduces support for Unix Domain Sockets.
Added the necessary include file to curl_addrinfo.c.

Note: The SDK (which is considered beta) has to be installed, VS 2017
project file has to be re-targeted for Windows 10.0.17061 and #define
enabled in config-win32.h.