Add license

This is important because this contains some code copied from nix (as an
interim expediency until that functionality can be exported via nix's
API).  The license specified here must be compatible with this borrowing.
Select the same license that nix is released under: lgpl2Plus.

nixos/modules/installer/tools/nixos-option/default.nix

@@ -4,4 +4,7 @@ stdenv.mkDerivation rec {
   src = ./.;
   nativeBuildInputs = [ cmake pkgconfig ];
   buildInputs = [ boost nix ];
+  meta {
+    license = stdenv.lib.licenses.lgpl2Plus;
+  }
 }