jsonnet-lint: filename context missing for shortcut function definitions

For long form functions, this works fine.
```
$ cat bar.jsonnet
local bar = function (str) { bar: str };
{}
$ jsonnet-lint bar.jsonnet
bar.jsonnet:1:7-40 Unused variable: bar

local bar = function (str) { bar: str };

Problems found!
```

Using short form definition misses the filename context in the message

```
$ cat foo.jsonnet
local dofoo(str) = { foo: str };
{}
$ jsonnet-lint foo.jsonnet
Unused variable: dofoo

Problems found!
```