cmake: clang: Enable -Waddress-of-packed-member

Several years ago we started enabling the -Waddress-of-packed-member
warning in commit cf111962 for GCC.

Do the same for Clang, since there is no reason not to and the warning
is very useful, especially in protocol stacks.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>