opengl: don't fail hard when distro is unknown

[ericLemanissier]

Specify library name and version: opengl/system

without this change, all unknown distribution have a hard fail with

opengl/system: WARN: Don't know how to install OpenGL for your distro.
opengl/system: ERROR: while executing system_requirements(): local variable 'packages' referenced before assignment
ERROR: Error in system requirements

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

[conan-center-bot]

All green in build 1 (71bb5df29631efe01f13acb113b4d2202bc32a6a)! 😊

• opengl/system: Generated 1 packages. All logs here

[ericLemanissier]

@jgsogo WDYT of this PR ?
Without it we have unavoidable hard failures on some distros: https://github.com/bincrafters/conan-qt/runs/866169432?check_suite_focus=true#step:10:523

[jgsogo]

If we want to fail for an unknown distro, let's do it, but not because a variable is not defined 👍