eventdev: add missing C++ guards

eventdev: add missing C++ guards

Some public header files were missing 'extern "C"' C++ guards,
and couldn't be used by C++ applications. Add the missing guards.

Fixes: dc39e2f359b5 ("eventdev: add ring structure for events")
Fixes: 7a3357205755 ("lib: remove C++ include guard from private headers")
Cc: stable@dpdk.org
Signed-off-by: Brian Dooley <brian.dooley@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>