intel/genxml: Make assert in gen_pack_header print a message.

intel/genxml: Make assert in gen_pack_header print a message.

Python's assert can take both a condition and a string, which will cause
it to print the string if the assertion trips.  (You can't use parens as
that creates a tuple.)  Doing "condition and string" works in C, but
doesn't have the desired effect in Python.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>