tests/gnrc_ipv6_nib_6ln: Fix error w/ clang on macOS

_is_reachable is only used when GNRC_IPV6_NIB_CONF_ARSM
is enabled, and as such it must be guarded so that clang
doesn't complain about a unused function in case
GNRC_IPV6_NIB_CONF_ARSM is not set

Similar to #7910
Relates to 6473