BUILD: memory: fix pointer declaration for atomic CAS

The calls to HA_ATOMIC_CAS() on the lockfree version of the pool allocator
were mistakenly done on (void*) for the old value instead of (void **).
While this has no impact on "recent" gcc, it does have one for gcc < 4.7
since the CAS was open coded and it's not possible to assign a temporary
variable of type "void".

No backport is needed, this only affects 1.9.
diff --git a/include/common/memory.h b/include/common/memory.h
index a0b6d62..da96b82 100644
--- a/include/common/memory.h
+++ b/include/common/memory.h
@@ -272,7 +272,7 @@
 	do {
 		*POOL_LINK(pool, ptr) = (void *)free_list;
 		__ha_barrier_store();
-	} while (!HA_ATOMIC_CAS(&pool->free_list, (void *)&free_list, ptr));
+	} while (!HA_ATOMIC_CAS(&pool->free_list, (void **)&free_list, ptr));
 	HA_ATOMIC_SUB(&pool->used, 1);
 }