sched_ext: Collect ext_*.c include headers in build_policy.c

Move <linux/btf_ids.h> from ext.c and "ext_idle.h" from ext.c (plus its
self-include in ext_idle.c) into build_policy.c. Subsequent patches add
their headers the same way for consistency.

No functional change.

Signed-off-by: Tejun Heo <tj@kernel.org>