raid6/test: fix a compilation warning

The compilation warning is redefination showed as following:

        In file included from tables.c:2:
        ../../../include/linux/export.h:180: warning: "EXPORT_SYMBOL" redefined
         #define EXPORT_SYMBOL(sym)  __EXPORT_SYMBOL(sym, "")

        In file included from tables.c:1:
        ../../../include/linux/raid/pq.h:61: note: this is the location of the previous definition
         #define EXPORT_SYMBOL(sym)

Fixes: 69a94abb ("export.h, genksyms: do not make genksyms calculate CRC of trimmed symbols")
Signed-off-by: Zhengyuan Liu <liuzhengyuan@kylinos.cn>
Signed-off-by: Song Liu <songliubraving@fb.com>