Added an external dependency on protobufs, included when SPIRV_BUILD_FUZZER is...

Added an external dependency on protobufs, included when SPIRV_BUILD_FUZZER is defined, so that they can be used by the (upcoming) spirv-fuzz tool.  Also updated the kokoro build scripts, for relevant targets, to clone an appropriate tag of the protobufs repo, and to pass -DSPIRV_BUILD_FUZZER to the configurations for which we intend to ultimately build spirv-fuzz. (#2616)