fix compile error: owned vector ~[] now immutable

```
rust-sdl2_image/src/sdl2_image/lib.rs:157:17: 157:33 error: type `~[<generic #7>]` does not implement any method in scope named `push`
rust-sdl2_image/src/sdl2_image/lib.rs:157                 used.push(*flag)
                                                          ^~~~~~~~~~~~~~~~
error: aborting due to previous error
```

fixed.