[pfff] make install-findlib pick up graph_code and ocamlgraph

Summary:

Looks like graph_code was missing a META file, and ocamlgraph was
referring to a missing `graph.a` without the `ocaml` prefix.

Test Plan:

From the root:

  make uninstall-findlib
  make install-findlib # includes graph_code and ocamlgraph; succeeds

Reviewers: pieter

Reviewed By: pieter

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Task ID: #

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