Enable -Wpragma-pack warnings as errors.

The use of pragma pack(1) affects the packed-ness of every struct
definition that follows it in the translation unit, even in following
header files. This can lead to multiple translation units disagreeing
on whether a struct is packed.

Enable the warnings (as errors) that help spot this problem.

Bug: http://b/73971777
Test: treehugger
Change-Id: Id3096f35dff5dc8723d30d1305f1ed7ae8190a32