lib/raid6/test: fix multiple definition linking error

GCC 10+ defaults to -fno-common, which enforces proper declaration of
external references using "extern". without this change a link would
fail with:

  lib/raid6/test/algos.c:28: multiple definition of `raid6_call';
  lib/raid6/test/test.c:22: first defined here

the pq.h header that is included already includes an extern declaration
so we can just remove the redundant one here.

Cc: <stable@vger.kernel.org>
Signed-off-by: Dirk Müller <dmueller@suse.de>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Song Liu <song@kernel.org>

lib/raid6/test/test.c

@@ -19,7 +19,6 @@
 #define NDISKS		16	/* Including P and Q */
 
 const char raid6_empty_zero_page[PAGE_SIZE] __attribute__((aligned(PAGE_SIZE)));
-struct raid6_calls raid6_call;
 
 char *dataptrs[NDISKS];
 char data[NDISKS][PAGE_SIZE] __attribute__((aligned(PAGE_SIZE)));