Structs within slices or maps inside a marshalled struct are not checked

`musttag` does not report forgotten `A` struct annotations in the example below:

```go
package main

import "encoding/json"

type A struct {
	Name string
}

type B struct {
	Slice []A          `json:"slice"`
	Map   map[string]A `json:"map"`       
}

func main() {
	_, _ = json.Marshal(B{})
}
```