Build fails due to missing libxlt

1. Take master @ [e7649ba](https://github.com/daphne-eu/daphne/commit/e7649ba6bdc629a03936e49c27ed876efbce6db4)
2. `git clone --recursive https://github.com/daphne-eu/daphne.git`
3. `./build.sh`

Build fails with the following error:
```
[1/6] Linking CXX shared library release/libarrow.so.1300.0.0
FAILED: release/libarrow.so.1300.0.0
...
/usr/bin/ld: cannot find -lxtl: No such file or directory
collect2: error: ld returned 1 exit status
```

Start by checking out commit e7649ba and running the documented git clone --recursive and ./build.sh steps. Trace the linker failure for release/libarrow.so.1300.0.0 and the missing -lxtl dependency; done means the build completes successfully without the libxtl error.

Written by the indexing model from the issue text.