llama : suppress unref var in Windows MSVC (#8150)

* llama : suppress unref var in Windows MSVC

This commit suppresses two warnings that are currently generated for
src/llama.cpp when building on Windows MSVC

```console
C:\llama.cpp\src\llama.cpp(14349,45): warning C4101: 'ex':
unreferenced local variable [C:\llama.cpp\build\src\llama.vcxproj]
C:\llama.cpp\src\llama.cpp(19285,44): warning C4101: 'e':
unreferenced local variable [C:\llama.cpp\build\src\llama.vcxproj]
```

* Update src/llama.cpp

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>