Remove superfluous include of string.h from percpu.h

There's nothing in percpu.h that requires an explicit inclusion of
string.h.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>