Add missing headers to Arcs.h
Arcs.h is missing two includes for unordered_set and smtk/graph/ResourceBase.h. Because the symbols are missing something that just includes Arcs.h will fail to compile.
Arcs.h is missing two includes for unordered_set and smtk/graph/ResourceBase.h. Because the symbols are missing something that just includes Arcs.h will fail to compile.