db_update.cpp:  Include hardfork.hpp, fix non-unity build broken by #615 fix
db_update.cpp:  Include hardfork.hpp, fix non-unity build broken by #615 fix

pmconrad/graphene,bitsuperlab/cpp-play2,cryptonomex/graphene,cryptonomex/graphene,bigoc/openchain,pmconrad/graphene,peertracksinc/muse,bigoc/openchain,bitshares/bitshares-2,oxarbitrage/bitshares-core,oxarbitrage/bitshares-core,bitsuperlab/cpp-play2,bitshares/bitshares-2,bitshares/bitshares-2,pmconrad/graphene,bigoc/openchain,bitsuperlab/cpp-play2,peertracksinc/muse,oxarbitrage/bitshares-core,oxarbitrage/bitshares-core,peertracksinc/muse,peertracksinc/muse,pmconrad/graphene,bigoc/openchain,cryptonomex/graphene,bitshares/bitshares-2