boost: backport fix to make async_pipes work with asio

boost: backport fix to make async_pipes work with asio

async_pipe is missing the executor_type type, which is expected by
asio in /usr/include/boost/asio/impl/read.hpp. Without this, it's
not possible to even compile code that uses constructs such as:

boost::asio::io_service foo;
boost::process::async_pipe foopipe{ boost::process::async_pipe(foo) };

This is only relevant for Dunfell because master has already moved to
boost-1.73.0 in which this bug has been fixed. The bug is also not
present in Zeus, which uses boost-1.71.0.

Signed-off-by: Viktor Rosendahl <Viktor.Rosendahl@bmw.de>
Signed-off-by: Steve Sakoman <steve@sakoman.com>