common-licenses: update Libpng license text

common-licenses: update Libpng license text

The new libpng license comprises the terms and conditions from the zlib
license, and the disclaimer from the Boost license. The legacy libpng
license license, used until libpng-1.6.35, is appended to the
new license, following the precedent established in the Python Software
Foundation License version 2.

See for details:

https://sourceforge.net/projects/libpng/files/libpng16/1.6.36/

(From OE-Core rev: 673c2dcf1045f7264570f81e2880d7a03befd2af)

Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>