Header files in ostream exporter are not installed when building with cmake.

[owent]

Describe your environment
Header files in ostream exporter are not installed when building with cmake.

Steps to reproduce
Describe exactly how to reproduce the error. Include a code sample if applicable.

What is the expected behavior?
Header files installed.

What is the actual behavior?
Nothing in opentelemetry/exporters/ostream

Additional context
Tag: v0.7.0