Fix printf format string warning on MinGW.

Fix printf format string warning on MinGW.

Commit 517bf2d91 changed a printf format string to placate MinGW, which
at the time warned about "%lld".  Current MinGW is now warning about the
replacement "%I64d".  Reverting the change clears the warning on the
MinGW CI task, and hopefully it will clear it on build farm animal
fairywren too.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reported-by: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
Discussion: https://postgr.es/m/TYAPR01MB5866A71B744BE01B3BF71791F5AEA%40TYAPR01MB5866.jpnprd01.prod.outlook.com