package/botan: security bump to version 2.16

Fix a memory leak in GCM where if passed an unsuitable block cipher (eg
not 128 bit) it would throw an exception and leak the cipher object.

https://github.com/randombit/botan/blob/2.16.0/news.rst



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>