Expose thriftnb cmake target

[sbrandprog]

Auto generated ThriftConfig.cmake does not export thriftnb library, so it is impossible to use TNonblockingServer with CMake build system. Note that, library's export file (thriftnbTargets.cmake) is generated and correctly installed by CMake.

This commit adds conditional include of thriftnb's export file.

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes) - trivial change
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[sbrandprog]

Gentle ping for this merge request.