kernel/sched/fair: include missed header file, memory-tiers.h

kernel/sched/fair: include missed header file, memory-tiers.h

Commit f66f3c261487 ("mm/demotion: update node_is_toptier to work with
memory tiers") of the 'mm-unstable' tree moved the declaration of
'node_is_toptier()' from 'node.h' to 'memory-tiers.h' but didn't include
the header in a source file using the function, 'kernel/sched/fair.c'.
As a result, build fails as below:

    linux/kernel/sched/fair.c: In function `should_numa_migrate_memory':
    linux/kernel/sched/fair.c:1571:7: error: implicit declaration of function `node_is_toptier' [-Werror=implicit-function-declaration]
     1571 |      !node_is_toptier(src_nid)) {
          |       ^~~~~~~~~~~~~~~
    cc1: some warnings being treated as errors

This commit fixes the error by including the missed header file.

Link: https://lkml.kernel.org/r/20220820190720.248704-1-sj@kernel.org
Fixes: f66f3c261487 ("mm/demotion: update node_is_toptier to work with memory tiers") on mm-unstable
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>