Fix Cycles MetalRT error after recent specialization changes

intern/cycles/kernel/device/gpu/kernel.h

@@ -246,7 +246,7 @@ ccl_gpu_kernel(GPU_KERNEL_BLOCK_NUM_THREADS, GPU_KERNEL_MAX_REGISTERS)
 ccl_gpu_kernel_postfix
 
 #if defined(__KERNEL_METAL_APPLE__) && defined(__METALRT__)
-constant int __dummy_constant [[function_constant(0)]];
+constant int __dummy_constant [[function_constant(Kernel_DummyConstant)]];
 #endif
 
 ccl_gpu_kernel(GPU_KERNEL_BLOCK_NUM_THREADS, GPU_KERNEL_MAX_REGISTERS)

intern/cycles/kernel/device/metal/function_constants.h

@@ -2,6 +2,7 @@
  * Copyright 2021-2022 Blender Foundation */
 
 enum {
+  Kernel_DummyConstant,
 #define KERNEL_STRUCT_MEMBER(parent, type, name) KernelData_##parent##_##name,
 #include "kernel/data_template.h"
 };