gnutls: Fix failing ptests

When upgrading gnutls to the newest version 3.8.5, some ptest failed.
Backported a patch from upstream gnutls(not in any release yet) to
fix this issue.

Signed-off-by: Simone Weiß <simone.p.weiss@posteo.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>