schannel: fix compiler warnings

When UNICODE is not defined, the Curl_convert_UTF8_to_tchar macro maps
directly to its argument. As it is declared as a pointer to const and
InitializeSecurityContext expects a pointer to non-const, both MSVC and MinGW
issue a warning about implicitly casting away the const. Fix this by declaring
the variables as pointers to non-const.

Closes https://github.com/curl/curl/pull/1394