Turn some license strings into lib.licenses values

pkgs/development/libraries/amrwb/default.nix

@@ -36,6 +36,6 @@ stdenv.mkDerivation {
     # The wrapper code is free, but not the libraries from 3gpp.
     # It's a source code reference implementation with patents and licenses on
     # some countries, not redistributable.
-    license = "unfree";
+    license = stdenv.lib.licenses.unfree;
   };
 }