ITS#8383 Look for socklen_t in <ws2tcpip.h> too

MinGW targets do not have the <sys/socket.h> header. The configure check
would conclude that there is no socklen_t type, resulting in portable.h
containing its own definition of socklen_t, which would later conflict
with the actual definition in <ws2tcpip.h>.

Add <ws2tcpip.h> to the configure check for socklen_t, so that the
defined type is correctly detected.