raid6/test: fix a compilation error

The compilation error is redeclaration showed as following:

        In file included from ../../../include/linux/limits.h:6,
                         from /usr/include/x86_64-linux-gnu/bits/local_lim.h:38,
                         from /usr/include/x86_64-linux-gnu/bits/posix1_lim.h:161,
                         from /usr/include/limits.h:183,
                         from /usr/lib/gcc/x86_64-linux-gnu/8/include-fixed/limits.h:194,
                         from /usr/lib/gcc/x86_64-linux-gnu/8/include-fixed/syslimits.h:7,
                         from /usr/lib/gcc/x86_64-linux-gnu/8/include-fixed/limits.h:34,
                         from ../../../include/linux/raid/pq.h:30,
                         from algos.c:14:
        ../../../include/linux/types.h:114:15: error: conflicting types for ‘int64_t’
         typedef s64   int64_t;
                       ^~~~~~~
        In file included from /usr/include/stdint.h:34,
                         from /usr/lib/gcc/x86_64-linux-gnu/8/include/stdint.h:9,
                         from /usr/include/inttypes.h:27,
                         from ../../../include/linux/raid/pq.h:29,
                         from algos.c:14:
        /usr/include/x86_64-linux-gnu/bits/stdint-intn.h:27:19: note: previous \
        declaration of ‘int64_t’ was here
         typedef __int64_t int64_t;

Fixes: 54d50897 ("linux/kernel.h: split *_MAX and *_MIN macros into <linux/limits.h>")
Signed-off-by: Zhengyuan Liu <liuzhengyuan@kylinos.cn>
Signed-off-by: Song Liu <songliubraving@fb.com>