ospf6d: Recent changes in our build cause const to be respected

ospf6d: Recent changes in our build cause const to be respected

We are seeing this crash:

New LWP 7673]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `/usr/lib/frr/ospf6d -d -F datacenter -M snmp -A ::1'.
Program terminated with signal SIGABRT, Aborted.
(gdb) bt
    vtysh=vtysh@entry=0) at lib/command.c:1288
(gdb)

The command entered is `debug ospf6 lsa inter-router examin`.  Code
inspection leads us to the fact that FRR is declaring the data as
const but we are attempting to modify it, causing the crash.

Remvoe the const of this set/get and let things work.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>