GEOS-Chem submod update: Merge PR #1929 (set missing GCHP adv spc to …

…bkgrnd)

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

d54f88ab9 Merge PR #1929 (Set GCHP adv species not in restart to bkgrnd values)

This implements a bug fix so that all species (advected or non-advected)
are set to backround values if not present in the restart file.

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