Fix "warning: no previous prototype for function 'its_*_eval'".
* gettext-tools/src/its.c (its_translate_rule_eval,
its_localization_note_rule_eval, its_element_within_text_rule_eval,
its_preserve_space_rule_eval, its_extension_context_rule_eval,
its_extension_escape_rule_eval): Make 'static'.
