knownhost.c: Fixed warning that pointer targets differ in signedness

src/knownhost.c

@@ -427,7 +427,8 @@ knownhost_check(LIBSSH2_KNOWNHOSTS *hosts,
                            we can't match it */
                         break;
                     }
-                    libssh2_hmac_sha1_init(&ctx, node->salt, node->salt_len);
+                    libssh2_hmac_sha1_init(&ctx, (unsigned char *)node->salt,
+                                           node->salt_len);
                     libssh2_hmac_update(ctx, (unsigned char *)host,
                                         strlen(host));
                     libssh2_hmac_final(ctx, hash);