Define helper macros before they're used.

The code works if they're defined afterwards, but defining them earlier
makes it a bit clearer that they're helper macros used as components of
the macros we're exporting.

Change-Id: I326894eea67d2db43fedf4567699cd9eaaa0e004
Reviewed-on: https://code.wireshark.org/review/5956
Reviewed-by: Guy Harris <guy@alum.mit.edu>