spice-gtk: fix build for withPolkit==false case

The spice-gtk/default.nix expression provides an option, withPolkit,
to decide whether or not the library should be compiled with support
for polkit.  Currently setting that argument to false results in a
build failure:

  Run-time dependency polkit-gobject-1 found: NO (tried pkgconfig)
  meson.build:207:0: ERROR: Dependency "polkit-gobject-1" not found, tried pkgconfig

It appears that at some point spice-gtk began requiring the meson flag
-Dpolkit=disabled in this case.  This commit adds that flag when
!withPolkit.

pkgs/development/libraries/spice-gtk/default.nix

@@ -124,6 +124,8 @@ stdenv.mkDerivation rec {
   mesonFlags = [
     "-Dusb-acl-helper-dir=${placeholder "out"}/bin"
     "-Dusb-ids-path=${hwdata}/share/hwdata/usb.ids"
+  ] ++ lib.optionals (!withPolkit) [
+    "-Dpolkit=disabled"
   ];
 
   meta = with lib; {