last: fix false positive compiler warning

last: fix false positive compiler warning

login-utils/last.c: In function ‘list’:
login-utils/last.c:398:36: warning: argument to ‘sizeof’ in ‘strncat’ call
is the same expression as the source; did you mean to use the size of the
destination?  [-Wsizeof-pointer-memaccess]
  strncat(utline, p->ut_line, sizeof(p->ut_line));

The sizeof(utline) is defined as sizeof(p->ut_line) + 1, so the compiler got
that wrong.  Lets truncate strncat() otherway around to keep gcc 8.1 happy.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>