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
```

* Add link to GitHub Issue