Fix format modifier used in elog.

The previous commit 65014000b3 changed the variable passed to elog
from an int64 to a size_t variable, but neglected to change the modifier
in the format string accordingly.

Per failure on buildfarm member lapwing.

src/backend/utils/sort/tuplesort.c

@@ -2911,7 +2911,7 @@ mergeruns(Tuplesortstate *state)
 	USEMEM(state, state->availMem);
 #ifdef TRACE_SORT
 	if (trace_sort)
-		elog(LOG, "worker %d using " INT64_FORMAT " KB of memory for tape buffers",
+		elog(LOG, "worker %d using %zu KB of memory for tape buffers",
 			 state->worker, state->tape_buffer_mem / 1024);
 #endif