databases/mongodb36: unbreak with libc++ 7

In file included from src/third_party/asio-master/asio/src/asio.cpp:11:
In file included from src/third_party/asio-master/asio/include/asio/impl/src.hpp:44:
In file included from src/third_party/asio-master/asio/include/asio/detail/impl/reactive_descriptor_service.ipp:25:
In file included from src/third_party/asio-master/asio/include/asio/detail/reactive_descriptor_service.hpp:24:
In file included from src/third_party/asio-master/asio/include/asio/buffer.hpp:27:
In file included from src/third_party/asio-master/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