added prefix include dir

When I tried to install `cutorch` (as a dependency of `cunn`), it failed with this error:
```
/tmp/luarocks_cutorch-scm-1-2002/cutorch/lib/THC/THCStorage.h:4:23: fatal error: THStorage.h: No such file or directory
 #include "THStorage.h"
```

Torch is installed in my home directory.
I added the include dir containing `THStorage.h` in `CMakeLists.txt`. Not sure if this is the right fix, but it works for me.