generator: Don't use ocamldep -all option in this directory.

ocamldep -all (introduced in commit a3881445efd0) creates a dependency
rule

  optgroups.cmi ... : utils.cmx ...

Because we never build a native code version of the generator,
utils.cmx can never be remade, and so this results in optgroups.cmi
being always rebuilt and hence rebuilding the whole directory.

It's unclear how to fix this, but reverting the ocamldep -all change
in this directory works around it.

Partially reverts commit a3881445efd04d5fe6da29db8b2d62a30088b9d1.