build gravityDBfile_default like the other variables

seems this one was forgotten

Signed-off-by: Michael Woolweaver <michael@woolweaver.bid>

gravity.sh

@@ -58,7 +58,7 @@ fi
 # Set this only after sourcing pihole-FTL.conf as the gravity database path may
 # have changed
 gravityDBfile="${GRAVITYDB}"
-gravityDBfile_default="/etc/pihole/gravity.db"
+gravityDBfile_default="${piholeDir}/gravity.db"
 gravityTEMPfile="${GRAVITYDB}_temp"
 gravityDIR="$(dirname -- "${gravityDBfile}")"
 gravityOLDfile="${gravityDIR}/gravity_old.db"