libassuan: upgrade 2.5.5 -> 2.5.6

Changelog:
===========
 * Fix logging of confidential data.  [rA0fc31770fa]
 * Fix memory wiping.  [T5977]
 * Fix macOS build problem.  [T5440,T5610]
 * Upgrade autoconf stuff.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>