fbcode_builder: use build interface generator expression for thrift include directories
fbcode_builder: use build interface generator expression for thrift include directories

Summary:
Fix an error about source directories being included in the include
path in add_thrift_cpp2_library. I don't know why this was failing for
fb303 and not for other projects, but adding a generation expression
appears to fix the issue.

Reviewed By: strager

Differential Revision: D15725024

fbshipit-source-id: 6c02fed6c6703733cf9e0b130c0f90b70e3ea300

ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,phoad/rsocket-cpp