sched/psi: make psi_cgroups_enabled static

The static key psi_cgroups_enabled is only used inside file psi.c.
Make it static.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Link: https://lore.kernel.org/r/20230525103428.49712-1-linmiaohe@huawei.com

kernel/sched/psi.c

@@ -140,7 +140,7 @@
 static int psi_bug __read_mostly;
 
 DEFINE_STATIC_KEY_FALSE(psi_disabled);
-DEFINE_STATIC_KEY_TRUE(psi_cgroups_enabled);
+static DEFINE_STATIC_KEY_TRUE(psi_cgroups_enabled);
 
 #ifdef CONFIG_PSI_DEFAULT_DISABLED
 static bool psi_enable;