Fix -Wdiscarded-qualifiers warnings caused by a newer libxml2.

Fix -Wdiscarded-qualifiers warnings caused by a newer libxml2.

Reported by Manish Tiwari <matiwari@redhat.com> in
<https://lists.gnu.org/archive/html/bug-gettext/2024-04/msg00001.html>.

* gettext-tools/src/format-kde-kuit.c (format_parse): Declare the local variable
err as 'const xmlError *'.
* gettext-tools/src/its.c (its_rule_list_add_from_file,
its_rule_list_add_from_string, its_rule_list_extract, its_merge_context_alloc):
Likewise.
* gettext-tools/src/locating-rule.c (locating_rule_match): Likewise.