cryptodev: fix clang C++ include

[ upstream commit 1763c91b06ca65bbb8516f47b97cffe1cac97dde ]

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: c0f87eb ("cryptodev: change burst API to be crypto op oriented")
Fixes: d2a4223 ("cryptodev: do not store pointer to op specific params")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>

lib/librte_cryptodev/rte_crypto.h

@@ -113,15 +113,24 @@ struct rte_crypto_op {
 	rte_iova_t phys_addr;
 	/**< physical address of crypto operation */
 
+/* empty structures do not have zero size in C++ leading to compilation errors
+ * with clang about structure/union having different sizes in C and C++.
+ * While things are clearer with an explicit union, since each field is
+ * zero-sized it's not actually needed, so omit it for C++
+ */
+#ifndef __cplusplus
 	__extension__
 	union {
+#endif
 		struct rte_crypto_sym_op sym[0];
 		/**< Symmetric operation parameters */
 
 		struct rte_crypto_asym_op asym[0];
 		/**< Asymmetric operation parameters */
 
+#ifndef __cplusplus
 	}; /**< operation specific parameters */
+#endif
 };
 
 /**