The version of ddcutil in Debian is being updated to release 2.1.4.
Shared library package libddcutil5 replaces libddcutil4.
The code in powerdevil should work unchanged.
In debian/control, consider adding:
Build-Depends: libddcutil-dev
Recommends: libddcutil4 (>= 1.4.1) | libddcutil5 (>= 2.1.4)
or
Suggests: libddcutil4 (>= 1.4.1) | libddcutil5 (>= 2.1.4)