mk: fix warnings when adding extra warning flags

mk: fix warnings when adding extra warning flags

Starting with commit 9aa2053c6e81493b23346ff4e387903560de5c81
EXTRA_CFLAGS is sometimes being passed to the compiler without
WERROR_FLAGS which can cause spurious warnings by the dozen,
for example with when compiling with EXTRA_CFLAGS="-Wformat-security":

cc1: warning: -Wformat-security ignored without -Wformat [-Wformat-security]

Passing WERROR_FLAGS to AUTO_CPU helper makes the warning flag usage
consistent throughout the codebase, silencing the warnings.

Fixes: 9aa2053c6e81 ("mk: influence CPU flags with user input")

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
Acked-by: Simon Kagstrom <simon.kagstrom@netinsight.net>