package/libftdi: add license

package/libftdi: add license

The COPYING.LIB license file contains the test of the LGPL-2.0, but the
source code itself explicitly refers to the GPL-2.1-only. Additionally,
parts of the library (src/ftdi_stream.c) are under the MIT license.

The C++ bindings are udner the GPL-2.0-only with an exception, which is
expressed in the LICENSE file.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr:
  - the library is under both GPL-2.1-only and MIT
  - the GPL-2.0-only only applies to the C++ bindings
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>