login: Add nonstring attributes to struct utmp, struct utmpx [BZ #24899]

Commit 7532837d ("The
-Wstringop-truncation option new in GCC 8 detects common misuses")
added __attribute_nonstring__ to bits/utmp.h, but it did not update
the parallel bits/utmpx.h header.  In struct utmp, the nonstring
attribute for ut_id was missing.