tests: add an headers test

This test checks that the protocol and library headers include only what
they are supposed to include. That is, that the core headers do not
include the protocol headers and that the core protocol headers do not
include the non core library headers.
The build process now generates core protocol headers, but they are only
used in the test and don't get installed.

Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>