psi: Fix "no previous prototype" warnings when CONFIG_CGROUPS=n

When CONFIG_CGROUPS is disabled psi code generates the following warnings:

kernel/sched/psi.c:1112:21: warning: no previous prototype for 'psi_trigger_create' [-Wmissing-prototypes]
    1112 | struct psi_trigger *psi_trigger_create(struct psi_group *group,
         |                     ^~~~~~~~~~~~~~~~~~
kernel/sched/psi.c:1182:6: warning: no previous prototype for 'psi_trigger_destroy' [-Wmissing-prototypes]
    1182 | void psi_trigger_destroy(struct psi_trigger *t)
         |      ^~~~~~~~~~~~~~~~~~~
kernel/sched/psi.c:1249:10: warning: no previous prototype for 'psi_trigger_poll' [-Wmissing-prototypes]
    1249 | __poll_t psi_trigger_poll(void **trigger_ptr,
         |          ^~~~~~~~~~~~~~~~

Change declarations of these functions in the header to provide the
prototypes even when they are unused.

Fixes: 0e94682b ("psi: introduce psi monitor")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220119223940.787748-2-surenb@google.com

include/linux/psi.h

@@ -25,18 +25,17 @@ void psi_memstall_enter(unsigned long *flags);
 void psi_memstall_leave(unsigned long *flags);
 
 int psi_show(struct seq_file *s, struct psi_group *group, enum psi_res res);
-
-#ifdef CONFIG_CGROUPS
-int psi_cgroup_alloc(struct cgroup *cgrp);
-void psi_cgroup_free(struct cgroup *cgrp);
-void cgroup_move_task(struct task_struct *p, struct css_set *to);
-
 struct psi_trigger *psi_trigger_create(struct psi_group *group,
 			char *buf, size_t nbytes, enum psi_res res);
 void psi_trigger_replace(void **trigger_ptr, struct psi_trigger *t);
 
 __poll_t psi_trigger_poll(void **trigger_ptr, struct file *file,
 			poll_table *wait);
+
+#ifdef CONFIG_CGROUPS
+int psi_cgroup_alloc(struct cgroup *cgrp);
+void psi_cgroup_free(struct cgroup *cgrp);
+void cgroup_move_task(struct task_struct *p, struct css_set *to);
 #endif
 
 #else /* CONFIG_PSI */