sched/psi: Select KERNFS as needed

Users of KERNFS should select it to enforce its being built, so
do this to prevent a build error.

In file included from ../kernel/sched/build_utility.c:97:
../kernel/sched/psi.c: In function 'psi_trigger_poll':
../kernel/sched/psi.c:1479:17: error: implicit declaration of function 'kernfs_generic_poll' [-Werror=implicit-function-declaration]
 1479 |                 kernfs_generic_poll(t->of, wait);

Fixes: aff0370 ("sched/psi: use kernfs polling functions for PSI trigger polling")
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Randy Dunlap <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Suren Baghdasaryan <[email protected]>
Link: lore.kernel.org/r/[email protected]

Author: rddunlap

init/Kconfig

@@ -629,6 +629,7 @@ config TASK_IO_ACCOUNTING
 
 config PSI
 	bool "Pressure stall information tracking"
+	select KERNFS
 	help
 	  Collect metrics that indicate how overcommitted the CPU, memory,
 	  and IO capacity are in the system.