lib/raid6/test: fix multiple definition linking error

lib/raid6/test: fix multiple definition linking error

BugLink: https://bugs.launchpad.net/bugs/1971497
commit a5359ddd052860bacf957e65fe819c63e974b3a6 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>