Fix error: old-style function definition

https://rubyci.s3.amazonaws.com/debian-riscv64/ruby-master/log/20220122T050018Z.log.html.gz#miniruby
```
compiling vm_trace.c
vm_trace.c: In function 'rb_vm_memsize_postponed_job_buffer':
vm_trace.c:1599:1: error: old-style function definition [-Werror=old-style-definition]
 1599 | rb_vm_memsize_postponed_job_buffer()
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

vm.c

@@ -2733,7 +2733,7 @@ ruby_vm_destruct(rb_vm_t *vm)
 
 size_t rb_vm_memsize_waiting_list(struct list_head *waiting_list); // process.c
 size_t rb_vm_memsize_waiting_fds(struct list_head *waiting_fds); // thread.c
-size_t rb_vm_memsize_postponed_job_buffer(); // vm_trace.c
+size_t rb_vm_memsize_postponed_job_buffer(void); // vm_trace.c
 size_t rb_vm_memsize_workqueue(struct list_head *workqueue); // vm_trace.c
 
 // Used for VM memsize reporting. Returns the size of the at_exit list by

vm_trace.c

@@ -1596,7 +1596,7 @@ rb_vm_memsize_workqueue(struct list_head *workqueue)
 // Used for VM memsize reporting. Returns the total size of the postponed job
 // buffer that was allocated at initialization.
 size_t
-rb_vm_memsize_postponed_job_buffer()
+rb_vm_memsize_postponed_job_buffer(void)
 {
     return sizeof(rb_postponed_job_t) * MAX_POSTPONED_JOB;
 }