devel/boost-libs: unbreak consumers with libressl

$ make WITH=ASIO -C www/nghttp2
[...]
In file included from asio_common.cc:25:
In file included from ./asio_common.h:32:
In file included from ../src/includes/nghttp2/asio_http2.h:37:
In file included from /usr/local/include/boost/asio/ssl.hpp:19:
In file included from /usr/local/include/boost/asio/ssl/context.hpp:786:
/usr/local/include/boost/asio/ssl/impl/context.ipp:232:27: error: no member named 'SSL_CTX_get_default_passwd_cb_userdata' in the global namespace; did you mean 'SSL_CTX_set_default_passwd_cb_userdata'?

void* cb_userdata = ::SSL_CTX_get_default_passwd_cb_userdata(handle_);
                    ~~^

/usr/local/include/openssl/ssl.h:1640:6: note: 'SSL_CTX_set_default_passwd_cb_userdata' declared here
void SSL_CTX_set_default_passwd_cb_userdata(SSL_CTX *ctx, void *u);

^

PR: 214793
Submitted by: matthew@reztek.cz
Obtained from: upstream (via Gentoo)
Approved by: portmgr blanket