Enforce Signature Header structure matches Signature

Use a macro when defining the Signature Header and Signature structures
since the fields are required to match for correctness. This helps avoid changing code in one place and not the other.

- PR build: https://buildbot.openinfosecfoundation.org/builders/ken-tilera/builds/30
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/ken-tilera-pcap/builds/36