Add mixing defaults entry for aero in GEFS

The defulats file for GEFS was missing an entry for aero to ensure
@field@ get parsed at expt setup time. This was added to gfs as
part of NOAA-EMC#3053, but gefs was missed.

parm/config/gefs/yaml/defaults.yaml

@@ -31,4 +31,6 @@ stage_ic:
   USE_ATM_ENS_PERTURB_FILES: "NO"
 ocn:
   MOM6_INTERP_ICS: "NO"
-
+# config.aero has just a system-specific path to add.
+# This is handled by the setup_expt.py, but it has to be told to write to it.
+aero: {}