powerpc/ptdump: Only enable PPC_CHECK_WX with STRICT_KERNEL_RWX

ptdump_check_wx() is called from mark_rodata_ro() which only exists
when CONFIG_STRICT_KERNEL_RWX is selected.

Fixes: 453d87f6 ("powerpc/mm: Warn if W+X pages found on boot")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/922d4939c735c6b52b4137838bcc066fffd4fc33.1578989545.git.christophe.leroy@c-s.fr

arch/powerpc/Kconfig.debug

@@ -371,7 +371,7 @@ config PPC_PTDUMP
 
 config PPC_DEBUG_WX
 	bool "Warn on W+X mappings at boot"
-	depends on PPC_PTDUMP
+	depends on PPC_PTDUMP && STRICT_KERNEL_RWX
 	help
 	  Generate a warning if any W+X mappings are found at boot.