licenses: Add Tcl/Tk license.

It is similar to the BSD license but has a few restrictions regarding government
use. But as I am not a lawyer, I'm not sure if that could be merged with bsd3,
so I'm listing it here separately just to be sure.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/lib/licenses.nix

@@ -175,6 +175,12 @@
     shortName = "Proprietary";
     fullName = "Proprietary (non redistributable) license";
   };
+
+  tcltk = {
+    shortName = "Tcl/Tk";
+    fullName = "Tcl/Tk license";
+    url = http://www.tcl.tk/software/tcltk/license.html;
+  };
   
   zlib = {
     shortName = "zlib";