net-p2p/mldonkey*: Fix build with Clang 6

src/utils/lib/CryptoPP.h:3941:26: error: ISO C++11 does not allow access declarations; use using declarations instead

BufferedTransformation::Get;
                        ^

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/mldonkey-3.1.5_2.log
http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/mldonkey-core-3.1.5_2.log
http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/mldonkey-gui-3.1.5_2.log

PR: 229691
Submitted by: mickey242@gmx.net