srtpdec: fix build when libsrtp1 is being used
Fixup for !8198 (merged) when libsrtp1 is being used.
Should probably just remove the libsrtp1 code paths though. Not sure if we want to do that also in the 1.24 branch, but even debian buster (old old stable, from 2019) has a new-enough libsrtp2 version.
cc @philn
This was discovered on the CI in the 1.22 branch backport (but I would guess that's just because the srtp2 dev package wasn't installed on the ci).