Add missing value for ${gravityDBfile_default}

Signed-off-by: DL6ER <dl6er@dl6er.de>

gravity.sh

@@ -59,6 +59,7 @@ fi
 # Set this only after sourcing pihole-FTL.conf as the gravity database path may
 # have changed
 gravityDBfile="${GRAVITYDB}"
+gravityDBfile_default="/etc/pihole/gravity.db"
 gravityTEMPfile="${GRAVITYDB}_temp"
 gravityDIR="$(dirname -- "${gravityDBfile}")"
 gravityOLDfile="${gravityDIR}/gravity_old.db"