https_getsock: provided for schannel backend as well

The function https_getsock was only implemented properly when USE_SSLEAY
or USE_GNUTLS is defined, but it is also necessary for USE_SCHANNEL.

The problem occurs when Curl_read_plain or Curl_write_plain returns
CURLE_AGAIN. In that case CURL_OK is returned to the multi-interface an
the used socket is set to state CURL_POLL_REMOVE and the easy-state is
set to CURLM_STATE_PROTOCONNECT. This is fine, because later the socket
should be set to CURL_POLL_IN or CURL_POLL_OUT via multi_getsock. That's
where https_getsock is called and doesn't return any sockets.