cryptodev: fix clang C++ include

cryptodev: 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/cryptodev/rte_crypto.h:127:2: 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.

Fixes: c0f87eb5252b ("cryptodev: change burst API to be crypto op oriented")
Fixes: d2a4223c4c6d ("cryptodev: do not store pointer to op specific params")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>