Use TEXT_DUMP_HEADER macro instead of hard-coded string in pg_backup_archiver.c

Hi,

I noticed that in pg_backup_archiver.c, RestoreArchive() writes the dump header
using a hard-coded string:

ahprintf(AH, "--\n-- PostgreSQL database dump\n--\n\n");

However, the macro TEXT_DUMP_HEADER (defined in the same file) already contains
exactly the same content. To keep the code consistent and maintainable, this
patch replaces the hard-coded string with the macro.

A hard-coded version of TEXT_DUMPALL_HEADER exists in pg_dumpall.c, but since
it spans multiple files, it is left untouched.

--
Regards,
Japin Li
ChengDu WenWu Information Technology Co., Ltd.