Regular '-I' directives must be passed to ${CPP} so that when
pre-processed, 'sched_ns.h' can find the files it includes in the proper
places, instead of relying on what is in ${WORLDTMP}, i.e., nothing if
'_includes' was not executed, which happens, e.g., when using 'make
kernel-toolchain' and then 'make buildkernel' without having executed
a 'make buildworld', or possibly some outdated copy not matching an
updated source.
Reported by: Claude Opus 5.5, imp
Fixes: 510331fa3763 ("sched: Automatically namespace schedulers' implementation functions")