build: Silence unwanted warnings

Signed-off-by: Simon McVittie <smcv@collabora.com>
4 jobs for master in 7 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
manual #38257
allowed to fail manual
build:arch
passed #38256
build:bionic

00:06:58

passed #38255
build:buster

00:07:14

passed #38254
build:stretch

00:06:52