Bug 11644: (follow-up) add FIXME

Bug 11644: (follow-up) add FIXME

Arithmetic with monetary amounts is a *hard* problem -- or if
not hard, at least super-fiddly, particularly since in theory
a Koha database could have to deal with dozens of currencies
if used by a large research library.  This patch adds a FIXME
implicitly commenting on the poor state of affairs of available
monetary calculation modules on CPAN.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 7efc10542be6d836c52cf598174e28f84a9d6470)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>