Update unrar to 5.8.1

Summary:
Upstream doesn't know what a changelog is...

Signed-off-by: Pierre-Yves <pyu@riseup.net>

Test Plan:

• unrar x test.rar
• open the archive from file-roller

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl

Differential Revision: https://dev.getsol.us/D7059