multimedia/pipewire: unbreak MEDIASESSION after ea9c61f86374

$ daemon -f pipewire
$ /usr/local/libexec/xdg-desktop-portal-wlr
[ERROR] - pipewire: couldn't connect to context
[ERROR] - xdpw: failed to initialize screencast
'impl->enter_count > 0' failed at ../spa/plugins/support/loop.c:362 loop_leave()

PR: 273258
Regressed by: https://gitlab.freedesktop.org/pipewire/pipewire/-/commit/01096bf6695a