fix multiple many-to-one relationships to a table

[thuan1412]

Issue

We have a bug when two tables have many-to-one relationship to another table.

table users {
  id bigint [pk, increment]
}

table items {
  id bigint [pk, increment]
  user_id bigint [ref: > users.id, not null]
}

table products {
  id bigint [pk, increment]
  user_id bigint [ref: > users.id, not null]
}

With the above example, dbml-go generates the User struct with this result:

type User struct {
	ID       int64     `json:"id" mapstructure:"id"`
	Products []Product `gorm:"foreignkey:user_id"`
}

The expected result is

type User struct {
	ID       int64     `json:"id" mapstructure:"id"`
	Items    []Item    `gorm:"foreignkey:user_id"`
	Products []Product `gorm:"foreignkey:user_id"`
}

This PR resolve the above issue.