compilation warning on Ubuntu 20.04

Just a minor thing:

make[6]: Entering directory '/home/thomson/devel/kea/kea-1.9.0-git/_build/sub/src/lib/asiolink'
  CXX      libkea_asiolink_la-addr_utilities.lo
../../../../../src/lib/asiolink/addr_utilities.cc: In function ‘isc::asiolink::IOAddress isc::asiolink::offsetAddress(const isc::asiolink::IOAddress&, uint64_t)’:
../../../../../src/lib/asiolink/addr_utilities.cc:396:9: warning: unused variable ‘offset_idx’ [-Wunused-variable]
  396 |     int offset_idx = 0;
      |         ^~~~~~~~~~