C pre-processor failed in ide-backend

This happens on the `io-streams` codebase. I can move the issue to `ide-backend` project if we think it's an up-stream problem. Wanted to check w/ @chrisdone here first though.

```
<unknown>:(0,0)-(0,0): 
ide-backend-server: phase `C pre-processor' failed (exitcode = 1)
```
