Disallow inclusion of files from src/conf into src/utils

Disallow inclusion of files from src/conf into src/utils

The utils code should stay separated from other code (except for very
well justified cases). Unfortunately commit 272769beccd7479c75e700a6cb
made it trivial to break the separation (and not get slapped by the
syntax-check rule) by adding -I src/conf to the CFLAGS for utils.

Remove this shortcut and except the two offenders from the syntax check
so that the codebase can be kept separated.