www/firefox: unbreak PipeWire support after 8691df3a5142

libpipewire is used on demand via dlopen(). Currently, only enabled
for WebRTC getUserMedia (screen capture) under Wayland.

https://searchfox.org/mozilla-release/rev/e2929d8cad28/third_party/libwebrtc/webrtc/modules/desktop_capture/screen_capturer_linux.cc#28

PR: 259621
(cherry picked from commit f0a3f8e66866cb022f8405a4a2772836a1f06de7)