[UR][Offload] Rename ol_kernel_handle_t -> ol_symbol_handle_t

The interface remains the same, but the type name is different.

Author: RossBrunton

unified-runtime/source/adapters/offload/kernel.hpp

@@ -77,6 +77,6 @@ struct ur_kernel_handle_t_ : RefCounted {
     }
   };
 
-  ol_kernel_handle_t OffloadKernel;
+  ol_symbol_handle_t OffloadKernel;
   OffloadKernelArguments Args{};
 };