raid6: guard the tables.c include of <linux/export.h> with __KERNEL__

The export directives for the tables are already emitted with __KERNEL__
guards, but the <linux/export.h> include is not, causing errors when
building the raid6test program. Guard this include too to fix the
raid6test build.
Signed-off-by: WANG Xuerui <git@xen0n.name>
Link: https://lore.kernel.org/r/20230731104911.411964-3-kernel@xen0n.nameSigned-off-by: Song Liu <song@kernel.org>

lib/raid6/mktables.c

@@ -56,7 +56,9 @@ int main(int argc, char *argv[])
 	uint8_t v;
 	uint8_t exptbl[256], invtbl[256];
 
+	printf("#ifdef __KERNEL__\n");
 	printf("#include <linux/export.h>\n");
+	printf("#endif\n");
 	printf("#include <linux/raid/pq.h>\n");
 
 	/* Compute multiplication table */