sycl: enable dpcpp nightly builds with oneMKL and oneDNN

[AD2605]

Currently if one tries to build the SYCL backend of LLaMA with nightly dpcpp with oneDNN and oneMKL libraries included in the release, it disables the oneDNN support because of GPU_VENDOR not being defined, and the CMake errors out with the following error:

CMake Error at ggml/src/ggml-sycl/CMakeLists.txt:112 (target_link_libraries):
  Target "ggml-sycl" links to:

    MKL::MKL_SYCL::BLAS

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

This is because the MKL's Cmake does not export the MKL_SYCL target when it's a nightly compiler.
This PR enables the same.