tiff: Fix LICENSE

The contents of the LICENSE.md file included in the current source
code package match those of libtiff license, which seems to have been
the case since 1999 commit
https://gitlab.com/libtiff/libtiff/-/commit/0ef31e1f62aa7a8b1c488a59c4930775ee0046e4
where it was added with filename COPYRIGHT and was then changed to
LICENSE.md in 2022 commit
https://gitlab.com/libtiff/libtiff/-/commit/fa1d6d787fc67a1eeb3abccb790b5bee969d424b



Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>