examples/vhost_blk: fix build with gcc 10

GCC 10 defaults to -fno-common, this means a linker error will now be
reported if the same global variable is defined in more than one
compilation unit.

Fixes: c19beb3f38cd ("examples/vhost_blk: introduce vhost storage sample")
Cc: stable@dpdk.org
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: David Marchand <david.marchand@redhat.com>