core-deps/graphviz.bst: Add 'dot -c' as integration command

Needed for dot to find its plugins. Similar to commit caab48f1.

freedesktop-sdk.bst:components/libtool.bst is necessary because dot -c uses libltdl.so which is part of libtool.bst.

graphviz will be used by core-deps/gtkmmm-3.bst and friends in a future MR where they build their documentation.

See issue #434 (closed).