LoongArch: Use flexible-array member instead of zero-length array

Eliminate the following coccicheck warning:
./arch/loongarch/include/asm/ptrace.h:32:15-21: WARNING use flexible-array member instead

Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Yushan Zhou <katrinzhou@tencent.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>