tools/check-version-history: skip test if lxml is not installed

tools/check-version-history: skip test if lxml is not installed

```
================================= 1070/1408 ==================================
test:         systemd:dist / check-version-history
start time:   07:55:05
duration:     0.03s
result:       exit status 1
...
----------------------------------- stderr -----------------------------------
Traceback (most recent call last):
  File "/mnt/work/systemd/upstream-fork/main/tools/check-version-history.py", line 6, in <module>
    import lxml.etree as tree
ModuleNotFoundError: No module named 'lxml'
==============================================================================
```

Follow-up for 3691e7fce70149ed19edc5d3003e2d7c936c147a