gtk-sharp-bean: add license, fix description

pkgs/development/libraries/gtk-sharp-beans/default.nix

···
   dontStrip = true;
 
   meta = with stdenv.lib; {
-    description = "gtk-sharp-beans binds some API from Gtk+ that isn't in Gtk# 2.12.x";
+    description = "Binds some API from Gtk+ that isn't in Gtk# 2.12.x";
     platforms = platforms.linux;
+    license = licenses.lgpl21;
   };
 }