Gets C warning when for loop has non-unit type

[Albert Yang]

class Main
  def f() : int
    1
  end

  def main() : unit
    for i <- [0 .. 10] do
      this.f()
    end
  end
end

Compiling emits C warning incompatible integer to pointer conversion assigning to 'void *' from 'int64_t' (aka 'long').