message: Update protobuf to 3.15.3
diff: @@ -36,7 +36,7 @@ endforeach()
  ExternalProject_add(
  protobuf
  - URL https://github.com/protocolbuffers/protobuf/releases/download/v3.13.0/protobuf-cpp-3.13.0.tar.gz
  + URL https://github.com/protocolbuffers/protobuf/releases/download/v3.15.3/protobuf-cpp-3.15.3.tar.gz
  PREFIX protobuf
  CMAKE_ARGS "${CMAKE_ARGS}"
  SOURCE_SUBDIR cmake
  