net-p2p/retroshare: Switch from devel/upnp to net/miniupnpc

RetroShare does not build with upnp >= 1.8. Upstream does not have
a solution for this yet other than suggesting to switch to using
miniupnpc as a workaround.

PR: 237406
Submitted by: tobik
Approved by: peter@netkey.at (maintainer)