PEP 445: fix hook_malloc() in example 3

Add missing declaration "void *ptr;"

pep-0445.txt

@@ -308,6 +308,7 @@ Example to setup hooks on all memory allocators::
     static void* hook_malloc(void *ctx, size_t size)
     {
         PyMemAllocator *alloc = (PyMemAllocator *)ctx;
+        void *ptr;
         /* ... */
         ptr = alloc->malloc(alloc->ctx, size);
         /* ... */