Fix detection of protobuf with onnxruntime_PREFER_SYSTEM_LIB on Linux (#4230)
The CMake module is FindProtobuf.cmake [1]. Thus the name should be
capitalized so that protobuf can be found on case-sensitive file
systems.
[1] https://github.com/Kitware/CMake/blob/v3.17.3/Modules/FindProtobuf.cmake