devel/libgtop: Fix build on 16.0-CURRENT

ugly patch to make build on freebsd

As with the lsof patch (bug #395414), this makes things build on
16-current, but I don't know if it breaks older supported releases.

Build tested on: (truckman)

14.3-RELEASE, 14.4-RELEASE amd64 and i386
15.0-RELEASE, 15.1-RC1 amd64
16.0-CURRENT amd64

PR: 295284