DllImport should not be prefixed with lib

[jasoncouture]

The usage of [DllImport("libllama")] results in the windows binaries being prefixed with "lib".
The native libraries in the NuGet packages have renamed DLLs. This makes it annoying to copy in llama binary releases for windows, for testing purposes. If the DLL imports were replaced with just [DllImport("llama")], this issue would not exist on windows, and on other platforms libllama.so and dylibs would work too, even with the lib prefix.

From: https://learn.microsoft.com/en-us/dotnet/standard/native-interop/native-library-loading

When running on Windows, the DLL is searched for in the following order:

nativedep
nativedep.dll (if the library name does not already end with .dll or .exe)

When running on Linux or macOS, the runtime will try prepending lib and appending the canonical shared library extension. On these OSes, library name variations are tried in the following order:

nativedep.so / nativedep.dylib
libnativedep.so / libnativedep.dylib
nativedep
libnativedep

[jasoncouture]

This change would require the NuGet llama native binaries get renamed back to llama.dll for windows.

The reason I chose to bring this up, is I'm annoyed by having to remember to rename the dll. I'm trying to get OpenCL/ROCm working, as I don't have, nor do I want an nvidia GPU.