include/libdecnumber: replace FSF postal address with licenses URL

Some of the GPLv2 boiler-plate still contained the
obsolete "51 Franklin Street" postal address.

Replace it with the canonical GNU licenses URL recommended by the FSF:
https://www.gnu.org/licenses/

Signed-off-by: Sean Wei <me@sean.taipei>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250613.qemu.patch.04@sean.taipei>
Signed-off-by: Thomas Huth <thuth@redhat.com>
9 files changed