add MSBuild project reference to gvc for dotgen

add MSBuild project reference to gvc for dotgen

This is step 1 of 4 in replacing ortho with gvc as a link dependency
for dotgen. It ensures the correct build order under all
circumstances, also when dotgen is built stand-alone. Just a link
dependency is not enough which does not cause the link dependency to
actually be built.

A project reference is a more modern way of creating a project
dependency than the centralized project dependencies in the solution
file. See https://stackoverflow.com/a/2317017/3122101.

Towards https://gitlab.com/graphviz/graphviz/-/issues/2096.