[cmake warning] Calling host function from device function in SymbolBits

[SteveBronder]

Summary

When compiling from master I'm seeing the warning

src/data/../common/compressed_iterator.h(37): warning: 
  calling a __host__ function from a __host__ __device__ function is not allowed

This seems to come from compressed_iterator.h

inline XGBOOST_DEVICE size_t SymbolBits(size_t num_symbols) {
  auto bits = std::ceil(log2(static_cast<double>(num_symbols)));
  return std::max(static_cast<size_t>(bits), size_t(1));
}

You include cmath at the top so I think this can be

inline XGBOOST_DEVICE size_t SymbolBits(size_t num_symbols) {
  auto bits = ceil(log2(static_cast<double>(num_symbols)));
  return max(static_cast<size_t>(bits), size_t(1));
}

Reproduce

git clone --recursive https://github.com/dmlc/xgboost
mkdir build
cd build
cmake .. -DUSE_CUDA=ON
make # wait for around 77%

I'm using gcc 8.3 though I'm guessing this is an everything thing

[trivialfis]

Fixed in #5453