CLEANUP: mark defproxy as const on parse tune.fail-alloc

This fixes a gcc warning about a missing const on defproxy for
mem_parse_global_fail_alloc.

This is needed since the commit :

018251667e4c95478ce0026f4d700e0420f8ce24
CLEANUP: config: make the cfg_keyword parsers take a const for the
defproxy
1 file changed