chore: simpler include guard style

[devjgm]

This change is 

[coryan]

Reviewed 57 of 57 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #144 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #144   +/-   ##
=======================================
  Coverage   95.81%   95.81%           
=======================================
  Files          83       83           
  Lines        3847     3847           
=======================================
  Hits         3686     3686           
  Misses        161      161

Impacted Files	Coverage Δ
google/cloud/grpc_utils/async_operation.h	100% <ø> (ø)	⬆️
google/cloud/internal/random.h	100% <ø> (ø)	⬆️
...cloud/testing_util/check_predicate_becomes_false.h	100% <ø> (ø)	⬆️
google/cloud/testing_util/chrono_literals.h	100% <ø> (ø)	⬆️
google/cloud/iam_binding.h	100% <ø> (ø)	⬆️
google/cloud/status_or.h	96.51% <ø> (ø)	⬆️
google/cloud/optional.h	97.56% <ø> (ø)	⬆️
google/cloud/testing_util/expect_future_error.h	100% <ø> (ø)	⬆️
google/cloud/internal/future_then_impl.h	100% <ø> (ø)	⬆️
google/cloud/future_generic.h	100% <ø> (ø)	⬆️
... and 20 more

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