mm/mmu_context, sched/core: Fix mmu_context.h assumption

Some architectures (such as Alpha) rely on include/linux/sched.h definitions
in their mmu_context.h files.

So include sched.h before mmu_context.h.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>