How to define a struct to parse list of objects resource in hcl file. Keep getting `panic: unsuitable DecodeExpression target: no cty.Type (no cty field tags)`

### HCL Template
```hcl
resource "test" "test-policy" {
src_security_zone = src_sz
dst_security_zone = dst_sz
name = "testpolicy"
acl_type = "pba"
applied_on = "dst"
approval_ticket = "20501"
ports = [{ approval_ticket = "23653", port_range = "9000", protocol = "tcp" }, { approval_ticket = "23653", port_range = "10504", protocol = "tcp" },]
}
```
I defined the structure for parsing the above HCL file as below.

```
type NaclPolicyPort struct {
PortRange string `hcl:"port_range,attr"`
Protocol string `hcl:"protocol,attr"`
ApprovalTicket string `hcl:"approval_ticket,attr"`
}
type NaclPolicyResource struct{
LabelNaclPolicy string `hcl:"name,label"`
LabelNaclPolicyName string `hcl:"name,label"`

Name string `hcl:"name,attr"`
SrcSecurityZone string `hcl:"src_security_zone,attr"`
DstSecurityZone string `hcl:"dst_security_zone,attr"`
AclType string `hcl:"acl_type,attr"`
AppliedOn string `hcl:"applied_on,attr"`
ApprovalTicket string `hcl:"approval_ticket,attr"`
Ports []NaclPolicyPort `hcl:"ports"`
}
```

### Expected behavior
What should have happened?
I get an error when my program tries to parse hcl:ports as below.
```panic: unsuitable DecodeExpression target: no cty.Type for main.NaclPolicyPort (no cty field tags)```

### Actual behavior
What actually happened?
I was expecting this to be parsed and I should have been able to get the ports.