Restore basebackup_progress_done() to preserve ABI

Restore basebackup_progress_done() to preserve ABI

Commit e7564ee8cdc, which fixed base backup progress reporting on
backup failure, removed the external function basebackup_progress_done()
because it was no longer used in core. When that change was backpatched
to v15, it introduced an ABI break, which was reported by buildfarm
member crake.

This commit restores basebackup_progress_done() to preserve ABI
compatibility, even though it is no longer used in core, rather than
updating the .abi-compliance-history file. Because external backup
tools may still call this function.

Per buildfarm member crake.

Reported-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CAD5tBcJ+ktrEp=PT8Gq-f=8mA2cDtZMB-hDMV4mMJ+9V46qBeQ@mail.gmail.com
Backpatch-through: 15-18