Fix missing include
Fix missing include

Add a missing include for std::string.

michele-brambilla/ConfigurationService,virtdb/cppzmq,chenwenbin928/cppzmq,bsn069/cppzmq,K0n63n/cppzmq,jens-auer/cppzmq,eburkitt/cppzmq,michele-brambilla/ConfigurationService,michele-brambilla/ConfigurationService,cmcmurrough/cppzmq,zeromq/cppzmq,michele-brambilla/ConfigurationService,zeromq/cppzmq,michele-brambilla/ConfigurationService,caidongyun/cppzmq