schannel: fix warning

- Fix warning 'integer from pointer without a cast' on 3rd arg in
  CertOpenStore. The arg type HCRYPTPROV may be a pointer or integer
  type of the same size.

Follow-up to e35b025.

Caught by Marc's CI builds.

lib/vtls/schannel.c

@@ -441,7 +441,8 @@ schannel_connect_step1(struct connectdata *conn, int sockindex)
         return result;
       }
 
-      cert_store = CertOpenStore(CURL_CERT_STORE_PROV_SYSTEM, 0, NULL,
+      cert_store = CertOpenStore(CURL_CERT_STORE_PROV_SYSTEM, 0,
+                                 (HCRYPTPROV)NULL,
                                  cert_store_name, cert_store_path);
       if(!cert_store) {
         Curl_unicodefree(cert_path);