Add boost header include for specializing boost::hash

[b1aafulei]

In include/fc/crypto/sha256.hpp we specialized boost::hash<> without properly including the Boost header <boost/container_hash/hash.hpp>.

fc/include/fc/crypto/sha256.hpp

Lines 125 to 130 in 0793fbe

	namespace boost
	{
	template<>
	struct hash<fc::sha256>
	{
	size_t operator()( const fc::sha256& s )const

This PR includes the above header.

The previous success of the fc build might have relied on some cross-inclusion among the Boost headers. However, while this is still okay with Boost 1.76, the fc library build starts to fail with the latest Boost 1.77 version (built on a Ubuntu 18.04 machine with g++-8) reporting the following errors:

.../external/fc/include/fc/crypto/sha256.hpp:127:12: error: ‘hash’ is not a class template
     struct hash<fc::sha256>
            ^~~~
.../external/fc/include/fc/crypto/sha256.hpp:128:5: error: explicit specialization of non-template ‘boost::hash’
     {
     ^

Considering that the current EOSIO software mostly depends on Boost 1.72, it is still okay for the fc library for the time being. But the problem might surface if we will upgrade to 1.77 or later.

[heifner]

This should target eosio