configure: Enable -Werror for MinGW builds, too

MinGW seems to compile currently without warnings, so it should
be safe to enable -Werror now for this environment, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1465373606-18486-1-git-send-email-thuth@redhat.com>
Tested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>