xgettext: PO: Fix error when a --exclude-file option is present (regression 2023-03-10).
Reported by Nicolas Peugnet <n.peugnet@free.fr> via Santiago Vila
at <https://lists.gnu.org/archive/html/bug-gettext/2024-07/msg00030.html>.
* gettext-tools/src/x-po.c (extract_add_message): If the message is the header
entry and is among the messages to exclude, save the header charset from it
before discarding it.
* gettext-tools/tests/xgettext-po-5: New file, based on
gettext-tools/tests/xgettext-po-3.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
