Add missing prelude prefix in .elp.toml file

Summary:
Without this change, opening one of the Erlang module via the ELP language server would result in an error:
```
Error resolving recursive target pattern.
```

Reviewed By: TheGeorge

Differential Revision: D63978348

fbshipit-source-id: f48b97f41603abf1ff3c3e254d8c390c261512ff

erlang/common_test/.elp.toml

@@ -1,4 +1,4 @@
 [buck]
 build_deps = false
 enabled = true
-included_targets = ["//erlang/common_test/..."]
+included_targets = ["prelude//erlang/common_test/..."]