net-im/{mtxclient,nheko}: bump PORTREVISION

Recent change to nlohmann-json, e39220e9d03a4b, didn't bump
consumers so the build was failing on the package cluster
as a newly-compiled nheko (which picks up newer nlohmann-json
API) was being linked against an older mtxclient (which didn't
have them).

Header-only libraries don't solve everything, when their
consumers aren't header-only.

Reported by: pkg-fallout