chromium: Add missing dependency on libcap.

After the pulseaudio refactor in NixOS/nixpkgs@a2a3508, libcap is no
longer propagated to chromium anymore. And we need to have libcap for
the renderer sandbox.

Build log: https://hydra.nixos.org/build/21689759/nixlog/1/raw



What makes me wonder is that given that this was propagated by
pulseaudio noone either seemed to have disabled pulseaudio support for
Chromium or just didn't report the build failure.

Half-assed testing done against all channels, because it builds the
sandbox and we can't break an already broken build twice (or maybe we
can, who knows...).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>