sched.hh included elf.hh, just so it can refer to the elf::tls_data
type. But now that we have rcu.hh which includes sched.hh and therefore
elf.hh, if we wish to use rcu in elf.hh (we'll do this in a later patch),
we have an include loop mess.

So better not include elf.hh from sched.hh, and just declare the one
struct we need.

After sched.hh no longer includes elf.hh and the dozen includes that
it further included, we need to add missing includes to some of the
code that included sched.hh and relied on its implict includes.