Ignore SDK CPP build directory

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, press enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug

/kind cleanup

/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it:

test/sdk/cpp/sdk/ gets generated on CPP SDK testing, but depending on your dev path, may not get cleaned up (and you may not want it to be cleaned up).

So finally adding it to .gitignore rather than forcing myself to delete it every time.

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

N/A

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markmandel, roberthbailey

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [markmandel,roberthbailey]

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[google-oss-robot]

New changes are detected. LGTM label has been removed.

[agones-bot]

Build Succeeded 👏

Build Id: 48f05d33-b3c2-4ea9-9da2-9a3503a0e4d4

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.18.0-7670b6e
• image: gcr.io/agones-images/agones-ping:1.18.0-7670b6e
• Linux C++ SDK (build): agonessdk-1.18.0-7670b6e-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.18.0-7670b6e.zip

A preview of the website (the last 30 builds are retained):

• https://7670b6e-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2284/head:pr_2284 && git checkout pr_2284
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.18.0-7670b6e