qtmultimedia: restrict libpulseaudio to linux

libpulseaudio doesn't currently build on darwin after a recent
upgrade (NixOS#160097). When looking closely at the darwin build, it
appeared that it wasn't being used at all.

❯ nix log $(nix-build --no-out-link  channel:nixpkgs-unstable --argstr system x86_64-darwin -A qt512.qtmultimedia) | grep PulseAudio
Checking for PulseAudio >= 0.9.10... no
  PulseAudio ............................. no

❯ nix log $(nix-build --no-out-link  channel:nixpkgs-unstable --argstr system x86_64-darwin -A qt514.qtmultimedia) | grep PulseAudio
Checking for PulseAudio >= 0.9.10... no
  PulseAudio ............................. no

❯ nix log $(nix-build --no-out-link  channel:nixpkgs-unstable --argstr system x86_64-darwin -A qt515.qtmultimedia) | grep PulseAudio
Checking for PulseAudio >= 0.9.10... no
  PulseAudio ............................. no

pkgs/development/libraries/qt-5/modules/qtmultimedia.nix

@@ -15,8 +15,9 @@ qtModule {
   pname = "qtmultimedia";
   qtInputs = [ qtbase qtdeclarative ];
   nativeBuildInputs = [ pkg-config ];
-  buildInputs = [ gstreamer gst-plugins-base libpulseaudio ]
-    ++ lib.optional (stdenv.isLinux) alsa-lib
+  buildInputs = [ gstreamer gst-plugins-base ]
+    # https://github.com/NixOS/nixpkgs/pull/169336 regarding libpulseaudio
+    ++ lib.optionals stdenv.isLinux [ libpulseaudio alsa-lib ]
     ++ lib.optional (lib.versionAtLeast qtbase.version "5.14.0" && stdenv.isLinux) wayland;
   outputs = [ "bin" "dev" "out" ];
   qmakeFlags = [ "GST_VERSION=1.0" ];