pg_rewind: Close backup_label file descriptor.

This was a relatively harmless leak, as createBackupLabel() is only
called once per pg_rewind invocation.

Author: Michael Paquier
Reported-By: Michael Paquier
Discussion: CAB7nPqRnOw30gOXe2_SPLjh37bgm4V+txbYAPwoXb97nGQ297w@mail.gmail.com
Backpatch: 9.5, where pg_rewind was introduced

src/bin/pg_rewind/pg_rewind.c

@@ -584,6 +584,7 @@ createBackupLabel(XLogRecPtr startpoint, TimeLineID starttli, XLogRecPtr checkpo
 	/* TODO: move old file out of the way, if any. */
 	open_target_file("backup_label", true);		/* BACKUP_LABEL_FILE */
 	write_target_range(buf, 0, len);
+	close_target_file();
 }
 
 /*