riscv: Add header include guards to insn.h

Add header include guards to insn.h to prevent repeating declaration of
any identifiers in insn.h.

Fixes: edde5584

 ("riscv: Add SW single-step support for KDB")
Signed-off-by: Liao Chang <liaochang1@huawei.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Fixes: c9c1af3f

 ("RISC-V: rename parse_asm.h to insn.h")
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230129094242.282620-1-liaochang1@huawei.com


Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>