sched: move sched_clock before first use

Move sched_clock() up to stop warning: weak declaration of `sched_clock'
after first use results in unspecified behavior (if -fno-unit-at-a-time).

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Mike Travis <travis@sgi.com>
Cc: Ben Herrenschmidt <benh@kernel.crashing.org>
Cc: Linuxppc-dev@ozlabs.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>