spanner: fix build tag
spanner: fix build tag

Build tags can be preceded only by line comments, and must be followed by a blank
line.

Change-Id: I2ceedcdb596942950c7ea395e3a1f50b70cc9fa8
Reviewed-on: https://code-review.googlesource.com/16450
Reviewed-by: Sarah Adams <2b342d1c32491b4e7587e4a0e291a5f0f95d4167@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go