sha3.hpp: fix compile error with Boost 1.79

[linh2931]

Resolve AntelopeIO/leap#31

When compile benchmark/hash.cpp on Ubuntu 18.04 where Boost is 1.79, following error was reported:

In file included from ../benchmark/hash.cpp:2:
../libraries/fc/include/fc/crypto/sha3.hpp:105:8: error: 'hash' is not a class template
 struct hash<fc::sha3>
        ^~~~
../libraries/fc/include/fc/crypto/sha3.hpp:106:1: error: explicit specialization of non-template 'boost::hash'

Compiles on Ubuntu 20.04 and 22.04 were fine.

Fix is to include <boost/functional/hash.hpp> in sha3.hpp.