Suppress benign MSVC C4251 warnings.
The warnings that are suppressed come from Google Protocol Buffers code.
According to the readme file at
https://github.com/protocolbuffers/protobuf/blob/master/cmake/README.md
these C4251 warnings can be disabled.

Change-Id: I89f664fc8b64a460829ccdf8fc5122d6a58b1442