Completion: check for compdef's existence

[hsk81]

In .local/share/bash-completion/completions/avalanche.sh check for the existence of compdef, since otherwise every time a non-zsh shell is launched an error is thrown:

if command -v compdef &> /dev/null
then
    compdef _avalanche avalanche 
fi

[arturrez]

this is caused by spf13/cobra#1529