Fix compilation on Windows with WAL_DEBUG

This has been broken since b060dbe000 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

src/backend/access/transam/xlog.c

@@ -1029,8 +1029,10 @@ XLogInsertRecord(XLogRecData *rdata,
 
 		if (!debug_reader)
 			debug_reader = XLogReaderAllocate(wal_segment_size, NULL,
-											  XL_ROUTINE(), NULL);
-
+											  XL_ROUTINE(.page_read = NULL,
+														 .segment_open = NULL,
+														 .segment_close = NULL),
+											  NULL);
 		if (!debug_reader)
 		{
 			appendStringInfoString(&buf, "error decoding record: out of memory while allocating a WAL reading processor");