utils.h: add a missing include

This tiny addition was needed to be able to install on FreeBSD.
Close https://github.com/CZ-NIC/knot-resolver/pull/44
5 jobs for utils-include in 40 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
canceled build:linux:amd64 #73000
amd64 linux docker

passed build:linux:amd64 #15660
docker linux amd64

00:00:39

 
  Test
passed deckard:linux:amd64 #15662
docker linux amd64

00:00:40

canceled respdiff:linux:amd64 #15663
docker linux amd64 allowed to fail

00:00:01

passed test:linux:amd64 #15661
docker linux amd64

00:00:28