examples/eventdev: 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: 3d1b33e44ae2 ("examples/eventdev: move common data into pipeline common")
Cc: stable@dpdk.org
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Acked-by: David Marchand <david.marchand@redhat.com>