hide .so symbols, avoid 2+ protobuf link conflict.

lichao

2021-05-19 981ca0fa022524fd04af988bc798f2d697d31548

 src/shm.cpp

@@ -25,6 +25,7 @@
    mshm_t(open_or_create, name.c_str(), size, 0, AllowAll()),
    name_(name)
{
   pmutex_ = FindOrCreate<Mutex>("ShmSelfControlMutex0");
}

SharedMemory::~SharedMemory()