aarch64: Fix bits/utmp.h for GCC 8

The same suppression of -Wstringop-truncation warnings as in commit
7532837d but for aarch64.

2018-02-21  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* sysdeps/unix/sysv/linux/aarch64/bits/utmp.h (struct utmp): Use
	__attribute_nonstring__.