Add compile time check for go-cty

In v1.11.0, the go-cty package dropped support for encoding/gob. Gob support is used
by the SDK to serialize HCL2 object specs over the wire. If a plugin or Packer upgrades
their version of zclconf/go-cty to one that does not contain Gob support the plugin will build
but crash when trying to run a build on an HCL2 template. By adding this check, a consumer of the SDK
will fail to compile if they are using an unsupported version of go-cty. This check is being added
as a guard to prevent Packer SDK consumers from becoming out of sync with unsupported go-cty versions.

```
~> go get github.com/hashicorp/go-cty v1.13.0
~> make dev
rpc/cty_encode.go:15:24 cannot use cty.Value{} (value of type cty.Value) as type gob.GobEncoder
```

rpc/cty_encode.go

@@ -4,10 +4,16 @@
 package rpc
 
 import (
+	"encoding/gob"
+
 	"github.com/zclconf/go-cty/cty"
 	"github.com/zclconf/go-cty/cty/json"
 )
 
+// Test that cty types implement the gob.GobEncoder interface.
+// Support for encoding/gob was removed in github.com/zclconf/[email protected].
+var _ gob.GobEncoder = cty.Value{}
+
 // cty.Value is does not know how to encode itself through the wire so we
 // transform it to bytes.
 func encodeCTYValues(config []interface{}) ([]interface{}, error) {