randconfig testing reveals that DEBUG_SET_MODULE_RONX needs to depend on
MMU otherwise these build errors are observed:

kernel/built-in.o: In function `set_section_ro_nx':
kernel/module.c:1738: undefined reference to `set_memory_nx'
kernel/built-in.o: In function `set_page_attributes':
kernel/module.c:1709: undefined reference to `set_memory_ro'

This is because the pageattr functions are not built for !MMU configs as
they don't have page tables.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>