Fix build for ext-proc example

[terrytangyuan]

This fixes the following issues when building the binary:

> go build -o /ext-proc
cache/cache.go:90:2: cacheKey declared and not used
./main.go:99:17: undefined: rest
./main.go:104:2: clientset declared and not used
./main.go:104:20: undefined: kubernetes

[kfswain]

/lgtm

[k8s-ci-robot]

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