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 3691e7f

tools/check-version-history.py

@@ -3,7 +3,12 @@
 
 import os
 import sys
-import lxml.etree as tree
+
+try:
+    import lxml.etree as tree
+except ImportError as e:
+    print(str(e), file=sys.stderr)
+    sys.exit(77)
 
 _parser = tree.XMLParser(resolve_entities=False)
 tree.set_default_parser(_parser)