emulators/mame: unbreak with libc++ 7

In file included from ../../../../../src/emu/http.cpp:12:
In file included from ../../../../../src/lib/util/server_ws_impl.hpp:8:
In file included from ../../../../../src/osd/asio.h:34:
In file included from ../../../../../3rdparty/asio/include/asio.hpp:21:
In file included from ../../../../../3rdparty/asio/include/asio/basic_datagram_socket.hpp:20:
In file included from ../../../../../3rdparty/asio/include/asio/basic_socket.hpp:40:
In file included from ../../../../../3rdparty/asio/include/asio/detail/reactive_socket_service.hpp:22:
In file included from ../../../../../3rdparty/asio/include/asio/buffer.hpp:27:
In file included from ../../../../../3rdparty/asio/include/asio/detail/string_view.hpp:23:
/usr/include/c++/v1/experimental/string_view:11:2: error: "<experimental/string_view> has been removed. Use <string_view> instead."

PR: 230401
Reported by: antoine (via exp-run)