relax unused block warning for duck typing

if a method `foo` uses a block, other (unrelated) method `foo`
can receives a block. So try to relax the unused block warning
condition.

```ruby
      class C0
        def f = yield
      end

      class C1 < C0
        def f = nil
      end

      [C0, C1].f{ block } # do not warn
```

1 files changed, 1 insertions, 0 deletions

diff --git a/vm.c b/vm.c
index 3fb57e2eb6..cf2bed84e4 100644
--- a/vm.c
+++ b/vm.c
@@ -4254,6 +4254,7 @@ Init_BareVM(void)
     vm->negative_cme_table = rb_id_table_create(16);
     vm->overloaded_cme_table = st_init_numtable();
     vm->constant_cache = rb_id_table_create(0);
+    vm->unused_block_warning_table = st_init_numtable();
 
     // setup main thread
     th->nt = ZALLOC(struct rb_native_thread);