liburcu: platforms: linux -> unix

There's probably no reason to restrict it to Linux.
I can't test directly, but Hombrew does have the package.
My intention is to build knot-resolver for Darwin as well.

pkgs/development/libraries/liburcu/default.nix

···
     description = "Userspace RCU (read-copy-update) library";
     homepage = http://lttng.org/urcu;
     license = licenses.lgpl21Plus;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = [ maintainers.bjornfor ];
   };