scripted-diff: Avoid interface keyword to fix windows gitian build

Rename `interface` to `interfaces`

Build failure reported by Chun Kuan Lee <ken2812221@gmail.com>
https://github.com/bitcoin/bitcoin/pull/10244#issuecomment-379434756

-BEGIN VERIFY SCRIPT-

git mv src/interface src/interfaces
ren() { git grep -l "$1" | xargs sed -i "s,$1,$2,g"; }
ren interface/            interfaces/
ren interface::           interfaces::
ren BITCOIN_INTERFACE_    BITCOIN_INTERFACES_
ren "namespace interface" "namespace interfaces"

-END VERIFY SCRIPT-

src/interface/handler.cpp

@@ -1,33 +0,0 @@
-// Copyright (c) 2018 The Bitcoin Core developers
-// Distributed under the MIT software license, see the accompanying
-// file COPYING or http://www.opensource.org/licenses/mit-license.php.
-
-#include <interface/handler.h>
-
-#include <util.h>
-
-#include <boost/signals2/connection.hpp>
-#include <memory>
-#include <utility>
-
-namespace interface {
-namespace {
-
-class HandlerImpl : public Handler
-{
-public:
-    HandlerImpl(boost::signals2::connection connection) : m_connection(std::move(connection)) {}
-
-    void disconnect() override { m_connection.disconnect(); }
-
-    boost::signals2::scoped_connection m_connection;
-};
-
-} // namespace
-
-std::unique_ptr<Handler> MakeHandler(boost::signals2::connection connection)
-{
-    return MakeUnique<HandlerImpl>(std::move(connection));
-}
-
-} // namespace interface