Suppress "missing Optix" cmake warning

## Environment
* OS Version: Ubuntu
* Source or binary build?
either

## Description
* Expected behavior: Our CI should be green, warning-free. But since https://github.com/ignitionrobotics/ign-cmake/pull/159 we get warnings for missing dependencies. Since Optix is optional and we don't plan on running CI for it, I think we should disable these warnings.

* Actual behavior: There are [CMake warnings](https://build.osrfoundation.org/job/ignition_rendering-ci-pr_any-ubuntu_auto-amd64/1257/cmake/new/)

## Steps to reproduce

Build `ign-rendering` without Optix installed.

## Output

`-- Cannot build component [optix] - Missing: OptiX`