www/firefox: depend on libpci to qualify for WebRender by default

On Intel Skylake this enables WebRender for Xorg and Wayland (wlroots).
Xwayland isn't qualified yet but Firefox can be tricked by unsetting
WAYLAND_DISPLAY or simply switching to native via MOZ_ENABLE_WAYLAND.

PR: 255344
Reported by: Evgeniy Khramtsov (via gecko@ list)