Turn some license strings into lib.licenses values

pkgs/development/libraries/libmrss/default.nix

@@ -14,7 +14,7 @@ stdenv.mkDerivation {
   meta = {
     homepage = http://www.autistici.org/bakunin/libmrss/doc;
     description = "C library for parsing, writing and creating RSS/ATOM files or streams";
-    license = "LGPLv2";
+    license = stdenv.lib.licenses.lgpl2;
 
     platforms = stdenv.lib.platforms.all;
     maintainers = [ stdenv.lib.maintainers.viric ];