Fix printf format string warning on MinGW.

Fix printf format string warning on MinGW.

This is a back-patch of 1319997d to branches 14-17 to fix an old warning
about a printf type mismatch on MinGW, in anticipation of a potential
expansion of the scope of CI's CompilerWarnings checks.  Though CI began
in 15, BF animal fairwren also shows the warning in 14, so we might as
well fix that too.

Original commit message (except for new "Backpatch-through" tag):

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.

Backpatch-through: 14-17
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