Feature-gate voice example
Feature-gate voice example

When building without enabling the 'voice' feature, the voice example
would fail. Instead, feature-gate it off so that it at least compiles
with a "Voice not enabled" message.

acdenisSK/serenity,zeyla/serenity,zeyla/serenity.rs,Roughsketch/serenity,eLunate/serenity,Flat/serenity,Lakelezz/serenity