psi: fix "defined but not used" warnings when CONFIG_PROC_FS=n

When CONFIG_PROC_FS is disabled psi code generates the following
warnings:

  kernel/sched/psi.c:1364:30: warning: 'psi_cpu_proc_ops' defined but not used [-Wunused-const-variable=]
      1364 | static const struct proc_ops psi_cpu_proc_ops = {
           |                              ^~~~~~~~~~~~~~~~
  kernel/sched/psi.c:1355:30: warning: 'psi_memory_proc_ops' defined but not used [-Wunused-const-variable=]
      1355 | static const struct proc_ops psi_memory_proc_ops = {
           |                              ^~~~~~~~~~~~~~~~~~~
  kernel/sched/psi.c:1346:30: warning: 'psi_io_proc_ops' defined but not used [-Wunused-const-variable=]
      1346 | static const struct proc_ops psi_io_proc_ops = {
           |                              ^~~~~~~~~~~~~~~

Make definitions of these structures and related functions conditional
on CONFIG_PROC_FS config.

Link: https://lkml.kernel.org/r/20220119223940.787748-3-surenb@google.com
Fixes: 0e94682b

 ("psi: introduce psi monitor")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>