Bug#962209: fix ftbfs with glibc-2.31

[Matthias Klose]

Package: src:lwipv6
Version: 1.5a-5
Severity: important
Tags: sid bullseye

fix ftbfs with glibc-2.31

patch at
http://launchpadlibrarian.net/482821761/lwipv6_1.5a-5_1.5a-5ubuntu1.diff.gz