The ipv6 MLD group state string conversion function is broken (only used if KTR is turned on).
This merely adds the missing entry, to comply with the enum described on `sys/netinet6/mld6_var.h:43`:
```
/*
* MLD per-group states.
*/
#define MLD_NOT_MEMBER 0 /* Can garbage collect group */
#define MLD_SILENT_MEMBER 1 /* Do not perform MLD for group */
#define MLD_REPORTING_MEMBER 2 /* MLDv1 we are reporter */
#define MLD_IDLE_MEMBER 3 /* MLDv1 we reported last */
#define MLD_LAZY_MEMBER 4 /* MLDv1 other member reporting */
#define MLD_SLEEPING_MEMBER 5 /* MLDv1 start query response */
#define MLD_AWAKENING_MEMBER 6 /* MLDv1 group timer will start */
#define MLD_G_QUERY_PENDING_MEMBER 7 /* MLDv2 group query pending */
#define MLD_SG_QUERY_PENDING_MEMBER 8 /* MLDv2 source query pending */
#define MLD_LEAVING_MEMBER 9 /* MLDv2 dying gasp (pending last */
/* retransmission of INCLUDE {}) */
```
I first noticed the issue on a FreeBSD 13.1.