Remove boost warnings

Boost itself uses deprecated headers within its own dependency tree.
This largely doesn't effect bmcweb, and rarely (if ever) has been a
problem, so enable BOOST_ALLOW_DEPRECATED_HEADERS which suppresses the
internal warnings in boost.

Tested:
Built with option enabled, and saw no deprecated header warnings.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ib371084463e4b2fb661c226a81c466121cf1f2dd