Log Message:
pluma: fix builds with latest itstool & py-libxml2 combination

A (hopefully temporary) workaround to fix a regression where itstool
with py-libxml2 2.9.12 ends up causing a Python segfault. (This did not
occur with py-libxml2 2.9.10.) For now, disable the help file target
that triggers the issue. Addresses PR pkg/56229 from Andrius V.