math/saga: Fix return type

Import Sentinel-3 OLCI Scene: fix for clang (use correct return type).
Upstream forgot to cherry-pick patch [1] from master to saga-9.8 branch.

Version 9.9.0 in main includes the patch, so only 9.8.2 in 2025Q3 has
to be patched directly.

[1] https://sourceforge.net/p/saga-gis/code/ci/66325d322c4371eba838245d816361f208c925b5/

Reported by: pkg-fallout