This file is indexed.

/usr/include/lastlog.h is in libc6-dev 2.24-11+deb9u4.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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/* This header file is used in 4.3BSD to define `struct lastlog',
   which we define in <bits/utmp.h>.  */

#include <utmp.h>