security/libomemo: Add new port

libomemo implements OMEMO (XEP-0384 v0.3.0) in C. For more information
see https://github.com/gkdr/libomemo.

PR: 265966
Submitted by: Michael A. Oshin <micadeyeye at gmail.com>
Reported by: Michael A. Oshin <micadeyeye at gmail.com>