tools/ocaml: abi-check: Check properly.

tools/ocaml: abi-check: Check properly.

Fix a broken regexp which would mention `$/' when it ought to have
mentioned `$'.  The result would be that it would match lines like
    type some_ocaml_type = Thing | Other_Thing
but ignore everything but the type name, giving wrong answers.

Check that we check mentioned types.  Otherwise if we fail to spot
some suitable thing in the ocaml, we would just omit checking this
type !

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>