graphics/frei0r: Fix build failure with OpenCV4

After the update of OpenCV to v4.5.1 the build was broken. Some
important commits [1] after release v1.7.0 fixed the issues.

[1] https://github.com/dyne/frei0r/compare/v1.7.0...master

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D28436