discord: work around upstream bug via pulseaudio

upstream bug causes large ping/high cpu usage when when using voice chat
if pulseaudio is not available at runtime, even though the executable
only explicitly needs ALSA

after a fix hits upstream, I will make pulseaudio support optional and
return the default to alsa

https://github.com/crmarsh/discord-linux-bugs/issues/16

pkgs/applications/networking/instant-messengers/discord/default.nix

@@ -2,7 +2,7 @@
 , alsaLib, atk, cairo, cups, dbus, expat, fontconfig, freetype, gdk_pixbuf
 , glib, gnome2, gtk2, libnotify, libX11, libXcomposite, libXcursor, libXdamage
 , libXext, libXfixes, libXi, libXrandr, libXrender, libXtst, nspr, nss, libxcb
-, pango, systemd, libXScrnSaver, libcxx }:
+, pango, systemd, libXScrnSaver, libcxx, libpulseaudio }:
 
 stdenv.mkDerivation rec {
 
@@ -36,7 +36,7 @@ stdenv.mkDerivation rec {
 
         paxmark m $out/opt/Discord
 
-        wrapProgram $out/opt/Discord --prefix LD_LIBRARY_PATH : "$LD_LIBRARY_PATH:${libcxx}/lib:${systemd.lib}/lib"
+        wrapProgram $out/opt/Discord --prefix LD_LIBRARY_PATH : "$LD_LIBRARY_PATH:${libcxx}/lib:${systemd.lib}/lib:${libpulseaudio}/lib"
 
         ln -s $out/opt/Discord $out/bin/
         ln -s $out/opt/discord.png $out/share/pixmaps