[SYCL] Fix more unused variables warnings (#3352)

Author: romanovvlad

sycl/plugins/opencl/pi_opencl.cpp

@@ -540,6 +540,7 @@ pi_result piextContextCreateWithNativeHandle(pi_native_handle nativeHandle,
                                              pi_context *piContext) {
   (void)num_devices;
   (void)devices;
+  (void)ownNativeHandle;
   assert(piContext != nullptr);
   assert(ownNativeHandle == false);
   *piContext = reinterpret_cast<pi_context>(nativeHandle);

sycl/source/detail/kernel_bundle_impl.hpp

@@ -99,6 +99,7 @@ class kernel_bundle_impl {
   }
 
   kernel get_kernel(const kernel_id &KernelID) const {
+    (void)KernelID;
     throw sycl::runtime_error("Not implemented", PI_INVALID_OPERATION);
   }