Add JSON functions to lib/gvc/gvc.def
Add JSON functions to lib/gvc/gvc.def

These functions are already added to lib/gvc.def (used by the Visual
Studio build), now they are also added to lib/gvc/gvc.def (used by the
CMake build).

ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz