security/libtomcrypt: update to 1.18.1

Changes: https://github.com/libtom/libtomcrypt/blob/v1.18.1/changes

Please note that the PDF documentation is not distributed anymore. You can find
the latest version pre-built at the release page:
https://github.com/libtom/libtomcrypt/releases/tag/v1.18.1