[BUG] Struct field function pointers don't support function call convention

**Describe the bug**
```c
alias FileSystemMonitorCallback = fn*(const char*);

opaque struct FileSystemMonitorEntry {
    fn_ptr: FileSystemMonitorCallback;
}

fn main() {
    let e = FileSystemMonitorEntry{};

    // This doesn't work
    e.fn_ptr("Hello!");

   // You have to do this
   let cb = e.fn_ptr;
   cb("Hello!");
}
```