Add explicit include for std::function in rates.h

[xndcn]

It fails in my computer (Ubuntu 22.04 + CUDA 12.6.85 + GCC 12.3.0):

nvblox/nvblox/include/nvblox/utils/rates.h:71:34: error: ‘function’ in namespace ‘std’ does not name a template type
   71 | using GetTimestampFunctor = std::function<int64_t()>;