Fix breakage in WinCNG backend caused by introducing libssh2_hmac_ctx_init.

The macro was defined to nothing for the libgcrypt backend, but not for WinCNG.  This brings the latter into line with the former.

src/knownhost.c

@@ -417,9 +417,9 @@ knownhost_check(LIBSSH2_KNOWNHOSTS *hosts,
                        plain input to produce a hash to compare with the
                        stored hash.
                     */
+                    unsigned char hash[SHA_DIGEST_LENGTH];
                     libssh2_hmac_ctx ctx;
                     libssh2_hmac_ctx_init(ctx);
-                    unsigned char hash[SHA_DIGEST_LENGTH];
 
                     if(SHA_DIGEST_LENGTH != node->name_len) {
                         /* the name hash length must be the sha1 size or

src/wincng.h

@@ -149,6 +149,7 @@ typedef struct __libssh2_wincng_hash_ctx {
  */
 
 #define libssh2_hmac_ctx _libssh2_wincng_hash_ctx
+#define libssh2_hmac_ctx_init(ctx)
 #define libssh2_hmac_sha1_init(ctx, key, keylen) \
   _libssh2_wincng_hash_init(ctx, _libssh2_wincng.hAlgHmacSHA1, \
                             SHA_DIGEST_LENGTH, key, keylen)