wireless plugin should not crash on platforms it isn't supported

[russorat]

Relevant telegraf.conf:

[[inputs.wireless]]

System info:

mac, telegraf 1.9

Steps to reproduce:

1. configure the [[inputs.wireless]] plugin on a mac system
2. notice that telegraf fails to start

Expected behavior:

telegraf starts but prints a warning message that the plugin is not supported on this platform (similar to the temp plugin)

Actual behavior:

telegraf fails to start

Additional info:

[Include gist of relevant config, logs, etc.]