isisd: always print adj->sysid (clang 3.6 warning)

As any new compiler version, clang 3.6 has new warnings, one of these
being that it now warns for testing whether the address of an array will
be true.

Of course there is no point in this check for the sysid, so let's always
just print the sysid.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>