Fix golint failure in cloud-provider-gcp/cmd/auth-provider-gcp/app/getcredentials.go

[cici37]

Golint check shows some suggestions in getcredentials.go

Ref: #198

[cici37]

/assign @DangerOnTheRanger

Would you mind to add some comments on this file to resolve golint failures? Thank you

[k8s-ci-robot]

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In response to this:

/assign @DangerOnTheRanger

Would you mind to add some comments on this file to resolve golint failures? Thank you

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