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

sys/net/gnrc/network_layer/ipv6/nib/nib.c

@@ -540,6 +540,7 @@ static void _handle_nbr_adv(kernel_pid_t iface, const ipv6_hdr_t *ipv6,
     }
 }
 
+#if GNRC_IPV6_NIB_CONF_ARSM
 static inline bool _is_reachable(_nib_onl_entry_t *entry)
 {
     (void)entry; /* _get_nud_state() might just resolved to UNMANAGED as macro */
@@ -551,6 +552,7 @@ static inline bool _is_reachable(_nib_onl_entry_t *entry)
             return true;
     }
 }
+#endif
 
 #if GNRC_IPV6_NIB_CONF_QUEUE_PKT
 static gnrc_pktqueue_t *_alloc_queue_entry(gnrc_pktsnip_t *pkt)