schannel: fix MinGW compile break

Original MinGW's w32api has a sytax error in its definition of
CALG_TLS1PRF [0]. Don't use original MinGW w32api's CALG_TLS1PRF
until this bug [1] is fixed.

[0] https://osdn.net/projects/mingw/scm/git/mingw-org-wsl/blobs/d1d4a17e51a2b78e252ef0147d483267d56c90cc/w32api/include/wincrypt.h
[1] https://osdn.net/projects/mingw/ticket/38391

Fixes https://github.com/curl/curl/pull/2721#issuecomment-403636043
Closes https://github.com/curl/curl/pull/2728