sys/mdup.[ch]: Fix licensing notice to refer to LGPL2.

This is debris from when the original version of mdup was written
out-of-tree, and had the default GPL slapped on it.

For clarity: the `mdup' code in mLib has in fact always been released
under the terms of the GNU Library General Public License, version 2 or
(at your option) any later version.  It's just that the notice was
wrong.