Add legacy build directives necessary for some go versions

[mattcary]

/kind bug

What this PR does / why we need it:

Some go versions (at least 1.16) require the legacy +build comments in addition to the new go:build commands. At least, without this change the following error happens when running make with go1.16:

go build -mod=vendor -ldflags "-X main.vendorVersion=v1.0.2-0-g5d19516" -o bin/gce-pd-csi-driver ./cmd/
cmd/main.go:28:2: //go:build comment without // +build comment
pkg/gce-pd-csi-driver/gce-pd-driver.go:28:2: //go:build comment without // +build comment
make: *** [gce-pd-driver] Error 1
Makefile:29: recipe for target 'gce-pd-driver' failed

/release-note-none

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mattcary

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[mattcary]

/release-note-none

[saikat-royc]

/lgtm

[mattcary]

The integration test successfully built the driver. We know the rest of the integration tests will fail, so I'll force-push.