sched/headers: Remove <linux/sched.h> from <linux/sched/cpufreq.h>

Make the <linux/sched/cpufreq.h> file a self-contained header and
remove the <linux/sched.h> dependency: users of it either don't
need <linux/sched.h> - or have already included it.

This reduces the size of the header dependency graph.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>