ARM: 8747/1: make CONFIG_DEBUG_WX depend on MMU

Without CONFIG_MMU, this results in a build failure:

./arch/arm/include/asm/memory.h:92:23: error: initializer element is not constant
 #define VECTORS_BASE  vectors_base
arch/arm/mm/dump.c:32:4: note: in expansion of macro 'VECTORS_BASE'
  { VECTORS_BASE, "Vectors" },

arch/arm/mm/dump.c:71:11: error: 'L_PTE_USER' undeclared here (not in a function); did you mean 'VTIME_USER'?
   .mask = L_PTE_USER,
           ^~~~~~~~~~

Obviously the feature only makes sense with an MMU, so let's add the
dependency here.

Fixes: a8e53c15 ("ARM: 8737/1: mm: dump: add checking for writable and executable")
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>