pgsql: Fix compilation on Windows with WAL_DEBUG

Fix compilation on Windows with WAL_DEBUG

This has been broken since b060dbe0001a that has reworked the callback
mechanism of XLogReader, most likely unnoticed because any form of
development involving WAL happens on platforms where this compiles fine.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVF14WKQMFwcJ=3okVDhiXpuK5f7YdT+BdYXbbypMHqWA@mail.gmail.com
Backpatch-through: 13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7636725b922c8cd68f21d040f3542d3bce9c68a4

Modified Files
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src/backend/access/transam/xlog.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)