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>

configure

@@ -1882,7 +1882,6 @@ gcc_flags="-Wno-missing-include-dirs -Wempty-body -Wnested-externs $gcc_flags"
 gcc_flags="-Wendif-labels -Wno-shift-negative-value $gcc_flags"
 gcc_flags="-Wno-initializer-overrides -Wexpansion-to-defined $gcc_flags"
 gcc_flags="-Wno-string-plus-int $gcc_flags"
-gcc_flags="-Wno-error=address-of-packed-member $gcc_flags"
 # Note that we do not add -Werror to gcc_flags here, because that would
 # enable it for all configure tests. If a configure test failed due
 # to -Werror this would just silently disable some features,