tools/check-version-history: avoid DeprecationWarning with newer lxml (#36860)

We get the same warning thousands of times:
/work/src/tools/check-version-history.py:28: FutureWarning: This search
incorrectly ignores the root element, and will be fixed in a future
version. If you rely on the current behaviour, change it to

"./refsynopsisdiv/funcsynopsis/funcprototype/funcdef/function[.='udev_device_get_properties_list_entry']"

We also need to update the ignorelist to the new form.

.gitignore

@@ -9,6 +9,7 @@
 .deps/
 .venv/
 .mypy_cache/
+.mkosi-private/
 __pycache__/
 /*.gcda
 /*.gcno

tools/check-version-history.py

@@ -24,7 +24,7 @@ def find_undocumented_functions(pages, ignorelist):
 
         hist_section = pagetree.find("refsect1[title='History']")
         for func in pagetree.findall("//funcprototype/funcdef/function"):
-            path = f"/refsynopsisdiv/funcsynopsis/funcprototype/funcdef/function[.='{func.text}']"
+            path = f"./refsynopsisdiv/funcsynopsis/funcprototype/funcdef/function[.='{func.text}']"
             assert pagetree.findall(path) == [func]
 
             if (
@@ -40,7 +40,7 @@ def construct_path(element):
     tag = element.tag
 
     if tag == "refentry":
-        return ""
+        return "."
 
     predicate = ""
     if tag == "varlistentry":