sysutils/py-salt: Add missing cryptography dependancy

Add cryptography to PYTHON_USES.
Salt requires the cryptography library, and will happily
fill up /var/log/salt with backtraces in the minion log
if it doesn't have it.

PR: 284715
Approved by: 0mp (mentor)
Approved by: maintainer timeout
Differential Revision: https://reviews.freebsd.org/D50530