mm: remove duplicate include statement in mmu.c

asm/sections.h is included more than once, Remove the one that isn't
necessary.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lkml.kernel.org/r/1600088607-17327-1-git-send-email-tiantao6@hisilicon.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>