configure: Make -Waddress-of-packed-member warnings be errors

configure: Make -Waddress-of-packed-member warnings be errors

We have now managed to eradicate all the places in the codebase
that triggered clang's -Waddress-of-packed-member warning. Remove
the compiler flag that exempted it from our usual -Werror policy.
This will prevent any new problematic code being added in future.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190208132112.31493-1-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>