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>

Author: KatrinJo

arch/loongarch/include/asm/ptrace.h

@@ -29,7 +29,7 @@ struct pt_regs {
 	unsigned long csr_euen;
 	unsigned long csr_ecfg;
 	unsigned long csr_estat;
-	unsigned long __last[0];
+	unsigned long __last[];
 } __aligned(8);
 
 static inline int regs_irqs_disabled(struct pt_regs *regs)