Add flexgrid_read_mod.o to list of dependencies for hcoi_gc_main_mod.o

This is a minor fix for an error in compiling GEOS-Chem Classic that we noticed when compiling GEOS-Chem Classic with a parallel compilation option.
The compile process fails occasionally due to the Get_Met_Fields subroutine in HCOI_GC_Main_Mod requiring FlexGrid_Read_Mod in a use statement,
but this module was not included in the Makefile dependencies. This is fixed by this commit.

Signed-off-by: Jimmie Lin <jimmie.lin@gmail.com>

GeosCore/Makefile

@@ -669,7 +669,7 @@ hcoi_gc_main_mod.o          : hcoi_gc_main_mod.F90                           \
                               get_ndep_mod.o          drydep_mod.o           \
                               modis_lai_mod.o         fast_jx_mod.o          \
                               hcoi_gc_diagn_mod.o     tomas_mod.o            \
-                              hco_interface_mod.o
+                              hco_interface_mod.o     flexgrid_read_mod.o
 
 initialize.o                : initialize.F                                   \
                               diag_mod.o              diag03_mod.o           \