[Bazel] Change visibility

[GregBowyer]

Make it possible for people to use google benchmark as a workspace rule.

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This change is 

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[coveralls]

Coverage remained the same at 87.088% when pulling 85fe4d4 on GregBowyer:patch-1 into 0e39052 on google:bazel.

[AppVeyorBot]

✅ Build benchmark 532 completed (commit 0d9424605b by @GregBowyer)

[EricWF]

@GregBowyer Are you going to be able to accept the CLA?

[EricWF]

@GregBowyer Ping... This patch cannot be merged until the CLA is accepted.

[EricWF]

@GregBowyer Closing this because you haven't signed the CLA. Please re-open if/when you're reading to do that.