It looks like the developers didn’t know how to apply it to 1.6 either. It was in the code from which 1.6 was derived, but appears to have been backed out. An attempt to reapply it seems to have failed.
It will take me too long to track down exactly where it was taken out of the source code.
It is possible that there is an alternative solution.
The code that was inserted isn’t even in trunk.
Basically you need to do a search of version of chan_sip.c starting at r97203 and find out when the variable state_interface was removed from the code, then look at the log entry, to work out why and whether there was a replacement.
I’d suggest a binary chop.