isisd: Bind SRv6 SID Struct ops to TLV type

isisd: Bind SRv6 SID Struct ops to TLV type

General Sub-Sub-TLV processing functions (i.e., copy, format, free,
pack, and unpack) perform a lookup of the handler specific for a
Sub-Sub-TLV in the `tlv_table`, and then call the specific handler to
process the Sub-Sub-TLV.
This commit adds the handlers for the SRv6 Structure Sub-Sub-TLV (stored
in `subsubtlv_srv6_sid_structure_ops`) to the `tlv_table`.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>