dnstap.mk: fix race around dnstap.pb-c.h

Probably.  The problem sometimes appeared on osx Travis.
3 jobs for dnstap-make in 2 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #5182
docker linux amd64
build:linux:amd64

00:01:14

 
  Test
passed #5184
docker linux amd64
deckard:linux:amd64

00:01:27

passed #5183
docker linux amd64
test:linux:amd64

00:01:01