polkit: mark as linux-only

While polkit is supposedly cross-platform, the test suite requires
a Linux-specific package (to test the D-Bus functionality). Since the
build fails when disabling the test, this effectively limits the
platform to just Linux.

pkgs/development/libraries/polkit/default.nix

···
     homepage = "http://www.freedesktop.org/wiki/Software/polkit";
     description = "A toolkit for defining and handling the policy that allows unprivileged processes to speak to privileged processes";
     license = licenses.lgpl2Plus;
-    platforms = platforms.unix;
+    platforms = platforms.linux;
     maintainers = teams.freedesktop.members ++ (with maintainers; [ ]);
   };
 }