The previous style(s) used in most places here violates the C standard, and in
practice can trigger -Wreserved-id-macro in clang for client code. So, switch
to a conventional style that doesn't trigger warnings (and is supported by
tools like clang-tidy).
All users of POST_PACKED_STRUCTURE also include <jack/types.h>, so to
avoid code duplication, we move the definition into a single place.
Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>