fdo#77307: Compare external formula tokens correctly.

To avoid different external (add-in) tokens from being evaluated equal.

Change-Id: If78187901bdca7fe0da50442b0bb66e0650dc70f
(cherry picked from commit f9cc0daec26016722bf5260e4e2634e6dcfe25a2)
Reviewed-on: https://gerrit.libreoffice.org/8942
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>