schannel: avoid incompatible pointer warning

with clang-6.0:
```
vtls/schannel_verify.c: In function 'add_certs_to_store':
vtls/schannel_verify.c:212:30: warning: passing argument 11 of 'CryptQueryObject' from incompatible pointer type [-Wincompatible-pointer-types]
                              &cert_context)) {
                              ^
In file included from /usr/share/mingw-w64/include/schannel.h:10:0,
                 from /usr/share/mingw-w64/include/schnlsp.h:9,
                 from vtls/schannel.h:29,
                 from vtls/schannel_verify.c:40:
/usr/share/mingw-w64/include/wincrypt.h:4437:26: note: expected 'const void **' but argument is of type 'CERT_CONTEXT ** {aka struct _CERT_CONTEXT **}'
   WINIMPM WINBOOL WINAPI CryptQueryObject (DWORD dwObjectType, const void *pvObject, DWORD dwExpectedContentTypeFlags, DWORD dwExpectedFormatTypeFlags, DWORD dwFlags,
                          ^~~~~~~~~~~~~~~~
```
Ref: https://msdn.microsoft.com/library/windows/desktop/aa380264

Closes https://github.com/curl/curl/pull/2648

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