riscv: Add license to fence.h

Add a missing license to fence.h.
Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240729-riscv_fence_license-v1-1-7d5648069640@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/fence.h

+/* SPDX-License-Identifier: GPL-2.0-only */
 #ifndef _ASM_RISCV_FENCE_H
 #define _ASM_RISCV_FENCE_H