usign: update to latest Git HEAD

usign: update to latest Git HEAD

This update fixes usign signature verification on files with certain
file sizes triggering a bug in the shipped SHA-512 implementation.

5a52b37 sha512: fix bad hardcoded constant in sha512_final()
3e6648b README: replace unicode character
716c3f2 README: add reference to OpenBSD signify
86d3668 README: provide reference for ed25519 algorithm
939ec35 usign: main.c: describe necessary arguments for -G

Ref: https://forum.openwrt.org/t/signature-check-failed/41945
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 991dd5a89340367920315a3fd0390a7423e6b34a)