Remove `-Werror` as required flag

Downstream projects might not build with same toolchain, which might
trigger warnings. Instead of treating these warnings as errors, using
this patch they are only warnings.

Also, add `-Werror` to the build flags to our testing pipeline, because
we do not want to accidentally introduce new warnings by mistake

Signed-off-by: Emil Ohlsson <emil.ohlsson@arm.com>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>