Add missing no-ctfconvert to C generation rules

Stop running ctfconvert over generated C files in the kernel by marking
them with no-ctfconvert.

This fixes warnings from ctfconvert trying to parse C files:

ctfconvert: file.c: Couldn't read ehdr: Invalid argument

Reviewed by: emaste, mmel
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D28156