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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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7d5a740331bfcfce5f9482ea4f2093941e1f367c

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