ioat_spec.h: rename xor to avoid C++ keyword collision

	struct spdk_ioat_generic_hw_desc generic;

	struct spdk_ioat_dma_hw_desc dma;

	struct spdk_ioat_fill_hw_desc fill;

	struct spdk_ioat_xor_hw_desc xor;

	struct spdk_ioat_xor_hw_desc xor_desc;

	struct spdk_ioat_xor_ext_hw_desc xor_ext;

	struct spdk_ioat_pq_hw_desc pq;

	struct spdk_ioat_pq_ext_hw_desc pq_ext;