licensing: relicense C++ library code under LGPL-3.0-or-later

licensing: relicense C++ library code under LGPL-3.0-or-later

Using LGPL-2.1 for C++ library code is an issue raised several times on
the linux-gpio mailing list. Programs using C++ libraries often include
significant portions of code generated behind the scenes from C++ headers
(via templates, default implementations etc.).

Section 3 of LGPL-3.0 clarifies the use of code defined in or generated
from C++ headers so this changset proposes to use LGPL-3.0 for all C++
library code in libgpiod.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Kent Gibson <warthog618@gmail.com>