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()
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

1 files changed, 1 insertions, 1 deletions

diff --git a/vm.c b/vm.c
index cb7007ac13..4cede5e247 100644
--- a/vm.c
+++ b/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