eventdev: fix clang C++ include

eventdev: fix clang C++ include

When compiling on FreeBSD with clang and include checking enabled,
errors are emitted due to differences in how empty structs/unions are
handled in C and C++, as C++ structs cannot have zero size.

lib/eventdev/rte_eventdev.h:992:2: error:
union has size 0 in C, non-zero size in C++

Since the contents of the union are all themselves of zero size,
the actual union wrapper is unnecessary. We therefore remove it for C++
builds - though keep it for C builds for safety and clarity of
understanding the code. The alignment constraint on the union is
unnecessary in the case where the whole struct is aligned on a 16-byte
boundary, so we add that constraint to the overall structure to ensure
it applies for C++ code as well as C.

Fixes: 1cc44d409271 ("eventdev: introduce event vector capability")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>