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
diff --git a/meson.build b/meson.build
index 529b9cb..778ca84 100644
--- a/meson.build
+++ b/meson.build
@@ -251,7 +251,8 @@
 '-DBOOST_NO_TYPEID',
 '-DBOOST_COROUTINES_NO_DEPRECATION_WARNING',
 '-DBOOST_URL_STANDALONE',
-'-DBOOST_URL_HEADER_ONLY'
+'-DBOOST_URL_HEADER_ONLY',
+'-DBOOST_ALLOW_DEPRECATED_HEADERS'
 ]),
 language : 'cpp')