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: Ib89d1c907b2297294a8ed7e7d1c3e715000a0e09