GEOS-Chem submod update: Merge PR #1946 (Fix custom mech build issues)

This commit informs the GCClassic superproject about the following
commits that were pushed to the GitHub geoschem/geos-chem repository:

19e8576bb Merge PR #1946 (Fix build issues in GCClassic w/ the custom mechanism)

This fixes several issues and omissions in the KPP/custom directory
that prevented building the custom mechanism.  Also the mechanism
definition files custom.kpp/fullchem.kpp and custom.eqn/fullchem.eqn
have been synchronized (which had not been done recently).

Signed-off-by: Bob Yantosca <[email protected]>