sched/headers: Move struct pre-declarations to the beginning of the header

There's a random number of structure pre-declaration lines in
kernel/sched/sched.h, some of which are unnecessary duplicates.

Move them to the head & order them a bit for readability.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org