powerpc/debug: PTDUMP should depend on DEBUG_FS

CONFIG_PPC_PTDUMP currently selects CONFIG_DEBUG_FS. But CONFIG_DEBUG_FS
is user-selectable, so we shouldn't select it. Instead depend on it.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/Kconfig.debug

@@ -356,8 +356,7 @@ config FAIL_IOMMU
 
 config PPC_PTDUMP
         bool "Export kernel pagetable layout to userspace via debugfs"
-        depends on DEBUG_KERNEL
-        select DEBUG_FS
+        depends on DEBUG_KERNEL && DEBUG_FS
         help
 	  This option exports the state of the kernel pagetables to a
 	  debugfs file. This is only useful for kernel developers who are