Disable -Waddress-of-packed-member temporarily.

Bug: http://b/33566695

Taking the address of packed data members yields pointers that only have
a guaranteed alignment of 1. Since this is an external/ project, we
temporarily will disable this warning until a proper fix can be made in
the upstream version of the project.

Test: Builds successfully with updated toolchain.
Change-Id: I8be14bdd5600108de227a1ec16d86caac452ec85