thread.c (vm_living_thread_num): constify vm arg

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63307 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread.c

@@ -196,7 +196,7 @@ vm_check_ints_blocking(rb_execution_context_t *ec)
 }
 
 static int
-vm_living_thread_num(rb_vm_t *vm)
+vm_living_thread_num(const rb_vm_t *vm)
 {
     return vm->living_thread_num;
 }