bug: only install the required headers (googleapis/google-cloud-cpp-spanner#993)
bug: only install the required headers (googleapis/google-cloud-cpp-spanner#993)

The install rule was creating unexpected directories in the
`$PREFIX/include` path, such as
`google/cloud/spanner/integration_tests`. Fixed the install rules to
only install the headers associated with specific targets.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp