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
diff --git a/src/pool.c b/src/pool.c
index 03cb2ec..2c9c9ff 100644
--- a/src/pool.c
+++ b/src/pool.c
@@ -695,8 +695,8 @@
 
 /* config parser for global "tune.fail-alloc" */
 static int mem_parse_global_fail_alloc(char **args, int section_type, struct proxy *curpx,
-                                      struct proxy *defpx, const char *file, int line,
-                                      char **err)
+                                       const struct proxy *defpx, const char *file, int line,
+                                       char **err)
 {
 	if (too_many_args(1, args, err, NULL))
 		return -1;