Fix progress report of REINDEX INDEX

I (Álvaro) broke that in commit 6212276e -- forgot to set the
necessary flag.  Repair.

Author: Amit Langote
Discussion: https://postgr.es/m/CA+HiwqEaM2tV5awKhP1vSbgjQe_uXVU15Oi4sTgwgempwMiT8g@mail.gmail.com