Re-introduce EXO, NPO and NCIT into makefile

[matentzn]

Currently, they are skipped:

https://github.com/EnvironmentOntology/environmental-exposure-ontology/blob/master/src/ontology/ecto.Makefile

The only thing that needs to happen if someone wants to add them back to the dynamic import pipeline is to simply delete the respective mirror and import goals in the above Makefile extension. For example, if you delete the following, the imports will be refreshed again as they should:

imports/exo_import.owl:
	echo "!!!!!EXO mirror currently skipped, see: https://github.com/CTDbase/exposure-ontology/issues/11"
	
mirror/exo.owl:
	echo "!!!!!EXO currently skipped, see: https://github.com/CTDbase/exposure-ontology/issues/11"

Dependent tickets:

• Broken xref annotation property CTDbase/exposure-ontology#11