Fix warning

```
../../../../ext/rbs_extension/../../src/util/rbs_allocator.c: In function ‘rbs_allocator_init’:
../../../../ext/rbs_extension/../../src/util/rbs_allocator.c:67:18: warning: old-style function definition [-Wold-style-definition]
   67 | rbs_allocator_t *rbs_allocator_init() {
      |                  ^~~~~~~~~~~~~~~~~~
At top level:
```

Author: soutaro

src/util/rbs_allocator.c

@@ -64,7 +64,7 @@ static rbs_allocator_page_t *rbs_allocator_page_new(size_t payload_size) {
     return page;
 }
 
-rbs_allocator_t *rbs_allocator_init() {
+rbs_allocator_t *rbs_allocator_init(void) {
     rbs_allocator_t *allocator = malloc(sizeof(rbs_allocator_t));
 
     const size_t system_page_size = get_system_page_size();