lib/memregion.c: include memregion.h

lib/memregion.c: include memregion.h

This addresses the following sparse warning:

  lib/memregion.c:8:5: warning: symbol 'memregion_alloc' was not declared. Should it be static?
  lib/memregion.c:14:6: warning: symbol 'memregion_free' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200921142852.875312-1-yanaijie@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>