Summary:
Update ThriftCppLibrary.cmake to record that the output depends on the thrift
`cpp2` template files.  The `THRIFT_TEMPLATE_FILES` variable was defined in
D16062657 but doesn't appear to be used anywhere.  This uses it, and also
updates it to only match the C++ template files.

Reviewed By: wez

Differential Revision: D16738442

fbshipit-source-id: aa4097e9ec1d009ef1e3ee2efd03ffa19a82ed12