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.

---
 kernel/sched/fair.c | 1 +
 1 file changed, 1 insertion(+)

index d642e9ff2829..0e3e08a093d4 100644
@@ -40,6 +40,7 @@
 
 #include <linux/cpuidle.h>
 #include <linux/interrupt.h>
+#include <linux/memory-tiers.h>
 #include <linux/mempolicy.h>
 #include <linux/mutex_api.h>
 #include <linux/profile.h>

2.25.1