math/saga: Update to 7.7.1

This is a bugfix release with some small enhancements.

Submitted by: rhurlin (maintainer)
Approved by: arrowd (mentor)
Relnotes: https://sourceforge.net/p/saga-gis/wiki/Changelog%207.7.1/attachment/changelog_saga_7.7.1.txt
Differential Revision: https://reviews.freebsd.org/D26613