Enable better malloc/free checking in a few cases.
Pinpointed by gcc 13 warning:
warning: function might be candidate for attribute 'malloc' [-Wsuggest-attribute=malloc]
* gettext-tools/src/po-time.h: Include attribute.h.
(po_strftime): Mark with attribute 'malloc'.
* gettext-tools/src/write-po.h: Include attribute.h.
(make_range_description_string): Mark with attribute 'malloc'.
