libesmtp: actually correct license

pkgs/development/libraries/libesmtp/default.nix

···
   meta = with stdenv.lib; {
     homepage = http://brianstafford.info/libesmtp/index.html;
     description = "A Library for Posting Electronic Mail";
-    license = licenses.gpl3;
+    license = licenses.lgpl21;
   };
 }