math/saga: Update to 7.8.1

Bug fix release, now ready for PostGIS 3.x.x[1]:

[1] https://sourceforge.net/p/saga-gis/wiki/Changelog%207.8.1/attachment/changelog_saga_7.8.1.txt

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D27032