examples/l2fwd-event: fix build on RHEL 7.6

examples/l2fwd-event: fix build on RHEL 7.6

CC l2fwd_event_generic.o
.../l2fwd_event_generic.c: In function
  ‘l2fwd_rx_tx_adapter_setup_generic’:
.../l2fwd_event_generic.c:203:3: error: missing initializer for field
  ‘impl_opaque’ of ‘struct <anonymous>’
  [-Werror=missing-field-initializers]
   }
   ^
In file included from .../l2fwd_event_generic.c:10:0:
.../include/rte_eventdev.h:1057:12: note: ‘impl_opaque’ declared here
    uint8_t impl_opaque;
            ^

  CC l2fwd_event_internal_port.o
.../l2fwd_event_internal_port.c: In function
  ‘l2fwd_rx_tx_adapter_setup_internal_port’:
.../l2fwd_event_internal_port.c:201:3: error: missing initializer for
  field ‘impl_opaque’ of ‘struct <anonymous>’
  [-Werror=missing-field-initializers]
   }
   ^
In file included from .../l2fwd_event_internal_port.c:10:0:
.../include/rte_eventdev.h:1057:12: note: ‘impl_opaque’ declared here
    uint8_t impl_opaque;
            ^

Fixes: 50f05aa6ed9a ("examples/l2fwd-event: setup Rx/Tx adapter")

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>