Make rb_ec_set_vm_stack conformant to the C++11 requirement

Https://learn.microsoft.com/en-us/cpp/build/reference/zc-inline-remove-unreferenced-comdat?view=msvc-140
> If `/Zc:inline` is specified, the compiler enforces the C++11
> requirement that all functions declared inline must have a definition
> available in the same translation unit if they're used.

vm.c

@@ -3583,7 +3583,7 @@ thread_alloc(VALUE klass)
     return TypedData_Make_Struct(klass, rb_thread_t, &thread_data_type, th);
 }
 
-inline void
+void
 rb_ec_set_vm_stack(rb_execution_context_t *ec, VALUE *stack, size_t size)
 {
     ec->vm_stack = stack;