fix for GHC 7.8.2

I was having trouble building this module on GHC 7.8.2. I got this error message:

```
Building samtools-0.2.2...
Preprocessing library samtools-0.2.2...
[1 of 6] Compiling Bio.SamTools.LowLevel ( dist/build/Bio/SamTools/LowLevel.hs, dist/build/Bio/SamTools/LowLevel.o )

src/Bio/SamTools/LowLevel.chs:202:3:
    Not in scope: ‘unsafePerformIO’
```

I managed to fix the problem by importing unsafePerformIO in LowLevel.chs.