build: Don't build `iotune` unconditionally
build: Don't build `iotune` unconditionally

We compile Seastar unconditionally so that changes to Seastar files are
reflected in Scylla when it's built.

We don't need to unconditionally build `iotune` in the same way.

`iotune` is still listed as a build artifact, so it will be built if
`ninja` is invoked without a particular target.

However, building a specific target (like `ninja build/dev/scylla`) will
not build `iotune`.

Fixes #4165

Signed-off-by: Jesse Haber-Kucharsky <526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>
Message-Id: <9fb96a281580a8743e04d5dd11398be53960cb58.1558100815.git.526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla