proc: make config PROC_CHILDREN depend on PROC_FS

Commit 2e13ba54 ("fs, proc: introduce CONFIG_PROC_CHILDREN")
introduces the config PROC_CHILDREN to configure kernels to provide the
/proc/<pid>/task/<tid>/children file.

When one deselects PROC_FS for kernel builds without /proc/, the config
PROC_CHILDREN has no effect anymore, but is still visible in menuconfig.

Add the dependency on PROC_FS to make the PROC_CHILDREN option disappear
for kernel builds without /proc/.

Link: https://lkml.kernel.org/r/20220909122529.1941-1-lukas.bulwahn@gmail.com
Fixes: 2e13ba54 ("fs, proc: introduce CONFIG_PROC_CHILDREN")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>

Cc: Iago López Galeiras <iago@endocode.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/proc/Kconfig

@@ -92,6 +92,7 @@ config PROC_PAGE_MONITOR
 
 config PROC_CHILDREN
 	bool "Include /proc/<pid>/task/<tid>/children file"
+	depends on PROC_FS
 	default n
 	help
 	  Provides a fast way to retrieve first level children pids of a task. See