hide vm_ep_in_heap_p_

`make leaked-globals` points out that this function is leaked.  This has
not been detected in our CI because it is defined only when
VM_CHECK_MODE is nonzero.

Just make it static and everytihng goes well.

vm.c

@@ -156,7 +156,7 @@ VM_EP_IN_HEAP_P(const rb_execution_context_t *ec, const VALUE *ep)
     }
 }
 
-int
+static int
 vm_ep_in_heap_p_(const rb_execution_context_t *ec, const VALUE *ep)
 {
     if (VM_EP_IN_HEAP_P(ec, ep)) {