sycl: add find_package call for OpenCL

[AD2605]

The nightly dpcpp compiler recently stopped shipping libOpenCL.so.

So when one uses a nightly build of oneDNN built with the said nightly compiler, it links against the target OpenCL::OpenCL (see this line), and when using this build of oneDNN in llama, it would expect the target OpenCL::OpenCL to be available.

This PR simply adds the find_package(OpenCL) when using the nightly compiler to resolve the same.

[sgeor255]

Shouldn't the find_package call here https://github.com/uxlfoundation/oneDNN/blob/743e4273d18bcb0e733723bf9289f14e8366687f/cmake/SYCL.cmake#L123 handle this?

[AD2605]

Shouldn't the find_package call here https://github.com/uxlfoundation/oneDNN/blob/743e4273d18bcb0e733723bf9289f14e8366687f/cmake/SYCL.cmake#L123 handle this?

This is used when building oneDNN not when building llama,
So the built oneDNN when being used in LLaMa will expect this target to be available

[Rbiessy]

I think Svet makes a good point. This could be fixed in the generated oneDNN config file similarly to this PR: uxlfoundation/oneDNN#2768
It seems it should try to find the OpenCL package when SYCL is enabled too.

[NeoZhangJianyu]

This PR could resolve your issue during building oneDNN.
But it's not necessary for official llama.cpp.
llama.cpp use the .a and .so files of oneDNN in fact.

This PR should be updated to oneDNN build script, instead of llama.cpp.