Include x64 directory for CUDA DLLs on Windows

[ajparsons]

I needed to add this DLL for the library to work on my system (possibly to do with a new cuda toolkit version, unclear).

Possibly related to #2070.

If anyone else encounters this, you can work around by adding it before importing:

import os
if "CUDA_PATH" in os.environ:
    os.add_dll_directory(os.path.join(os.environ["CUDA_PATH"], "bin", "x64"))
import llama_cpp