Use -Werror on all platforms except buster

This should be a slightly better fix than 088528a5
13 jobs for buster in 50 minutes and 42 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #5052
shell
build:bionic

00:16:59

passed #5054
shell
build:buster

00:16:45

passed #5053
shell
build:stretch

00:15:54

passed #5048
shell
build:trusty

00:22:25

passed #5049
shell
build:trusty-cmake3.5.0

00:22:23

passed #5050
shell
build:xenial

00:11:03

passed #5051
shell
build:xenial-fsanitize

00:11:18

 
  Test
passed #5058
shell
test:bionic

00:04:28

passed #5060
shell
test:buster

00:05:01

passed #5059
shell
test:stretch

00:04:00

passed #5055
shell
test:trusty

00:05:59

passed #5056
shell
test:trusty-cmake3.5.0

00:06:17

passed #5057
shell
test:xenial

00:03:54