Turn more licenses into lib.licenses style

Should eval cleanly, as far as -A tarball tells me.

Relevant: issue #2999, issue #739

pkgs/applications/science/misc/fityk/default.nix

@@ -16,7 +16,7 @@ stdenv.mkDerivation {
 
   meta = {
     description = "Curve fitting and peak fitting software";
-    license = "GPL2";
+    license = stdenv.lib.licenses.gpl2;
     homepage = http://fityk.nieto.pl/;
     platforms = stdenv.lib.platforms.linux;
   };