resolve linking error

[ivg]

A Ppx_forest module was referenced by both forest_ppx library and
the ppx_forest executable. This created a loop in the dependency
graph, under which the topological sorting is not defined, consequently
this lead to a compilation command with an incorrect ordering of
libraries (forest_ppx came before the forest_parser).

[ivg]

My emacs is set up to remove trailing whitespaces, that's why the fix is so long, in fact it should be one line of code change, i.e., deleting one line. So, if this extra noise bothers you, then I can try to disable this feature, and resubmit. Alternatively, you can just fix it yourself, and close the PR :)

[jonathan-dilorenzo]

Awesome! Thanks again!