Redundant import when creating foreign #pointer

e.g.

```
module Test where

data Test
{#pointer *test as TestPtr foreign -> Test}
```

generates

```
module Test where

import qualified Foreign.ForeignPtr as C2HSImp
import qualified Foreign.Ptr as C2HSImp

data Test
type TestPtr = C2HSImp.ForeignPtr (Test)
```

The extra (unused) `Foreign.Ptr` import blows up on `-Werror`.