tools: fix building manpages with lxml 6.1.3

manpages build fails on Arch with new lxml:

Traceback (most recent call last):
  File "/work/src/tools/make-directive-index.py", line 169, in make_page
    _extract_directives(directive_groups, formatting, page)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/work/src/tools/make-directive-index.py", line 18, in _extract_directives
    t = xml_parse(page)
  File "/work/src/tools/xml_helper.py", line 25, in xml_parse
    doc = tree.parse(page, _parser)
  File "src/lxml/etree.pyx", line 3717, in lxml.etree.parse
  File "src/lxml/parser.pxi", line 2045, in lxml.etree._parseDocument
  File "src/lxml/parser.pxi", line 2071, in lxml.etree._parseDocumentFromURL
  File "src/lxml/parser.pxi", line 1962, in lxml.etree._parseDocFromFile
  File "src/lxml/parser.pxi", line 1231, in lxml.etree._BaseParser._parseDocFromFile
  File "src/lxml/parser.pxi", line 647, in lxml.etree._ParserContext._handleParseResultDoc
  File "src/lxml/parser.pxi", line 765, in lxml.etree._handleParseResult
  File "src/lxml/parser.pxi", line 689, in lxml.etree._raiseParseError
  File "../src/man/hostname.xml", line 5
lxml.etree.XMLSyntaxError: Entity 'entities' not defined, line 5, column 11

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/work/src/tools/make-directive-index.py", line 180, in <module>
    _xml = make_page(_template_path, _xml_files)
  File "/work/src/tools/make-directive-index.py", line 171, in make_page
    raise ValueError('failed to process ' + page) from e
ValueError: failed to process ../src/man/hostname.xml

Relying on XMLParser() defaults leaves the external parameter entity
used to load custom-entities.ent unresolved. Request entity resolution explicitly.

Follow-up for 1a13e31d27

(cherry picked from commit a019396d2c)
(cherry picked from commit 24238c133b)
(cherry picked from commit 04b713061c)
(cherry picked from commit bc129b86a9)

tools/xml_helper.py

@@ -15,7 +15,7 @@ class CustomResolver(tree.Resolver):
 
         return None
 
-_parser = tree.XMLParser()
+_parser = tree.XMLParser(resolve_entities=True)
 # pylint: disable=no-member
 _parser.resolvers.add(CustomResolver())