add comment explaining the cmake change

Author: AD2605

ggml/src/ggml-sycl/CMakeLists.txt

@@ -51,9 +51,19 @@ target_compile_options(ggml-sycl PRIVATE "-Wno-narrowing")
 # Link against oneDNN
 set(GGML_SYCL_DNNL 0)
 if(GGML_SYCL_DNN)
+
+# The nightly dpcpp compiler recently stopped shipping libOpenCL
+# When using a nightly build of oneDNN built with the said nightly compiler, it links against the target OpenCL::OpenCL 
+# For context,this line in the oneDNN CMake: 
+# https://github.com/uxlfoundation/oneDNN/blob/743e4273d18bcb0e733723bf9289f14e8366687f/cmake/SYCL.cmake#L127
+# and when using this build of oneDNN in llama, it would expect the target OpenCL::OpenCL to be available.
+# Hence the below find_package call is required. At the time of writing, the release compiler
+# ships libOpenCL,hence only finding the package when the compiler is clang (nightly). 
+
     if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
            find_package(OpenCL REQUIRED)
     endif()
+
     find_package(DNNL)
     if(DNNL_FOUND)
         if (NOT DEFINED DNNL_GPU_VENDOR)