last: do not use non-standard __UT_NAMESIZE

last: do not use non-standard __UT_NAMESIZE

In commit b22332dd4 (last: fix wtmp user name buffer overflow
[asan], 2019-01-13), we started to make sure that the `ut_user`
field of the `utmpx` struct is always NUL-terminated. The
implementation makes use of the `__UT_NAMESIZE` define to
determine the position of the last character in that array. The
problem is that this is a non-standard define that is not
necessarily available on non-glibc platforms.

As there is no standardized define, we should just use `sizeof`.
This fixes compilation on musl libc based systems.

Signed-off-by: Patrick Steinhardt <ps@pks.im>