De-constify sched.h

[PATCH] De-constify sched.h

This reverts commit a8972ccf ("sched:
constify sched.h")

 1) Patch doesn't change any code here, so gcc is already smart enough
    to "feel" constness in such simple functions.
 2) There is no such thing as const task_struct.  Anyone who think
    otherwise deserves compiler warning.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>