[PATCH] node: fix C++ compatibility errors (option 2)

[Bruce Richardson <bruce.richardson@intel.com>]

C++ does not allow zero-sized unions - they end up being of size 1-byte,
which leads to C/C++ compatibility issues, flagged by the compiler.

  lib/node/rte_node_mbuf_dynfield.h:78:2: error: union has size 0 in C,
size 1 in C++ [-Werror,-Wextern-c-compat]
   78 |         union {
	         |         ^
		 1 error generated.

Fix the error by omitting the persistent_data field when it is
zero-sized, since it's unusable. Any app using the field must already
specify a size for the persistent data.

Fixes: 746e8736da70 ("node: add global mbuf dynfield")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
 lib/node/rte_node_mbuf_dynfield.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/node/rte_node_mbuf_dynfield.h b/lib/node/rte_node_mbuf_dynfield.h
index b77fdbb94f..09254c585a 100644
--- a/lib/node/rte_node_mbuf_dynfield.h
+++ b/lib/node/rte_node_mbuf_dynfield.h
@@ -69,6 +69,7 @@ typedef struct rte_node_mbuf_overload_fields {
  * 2. Overloadable fields: Fields which can be repurposed by two adjacent nodes.
  */
 typedef struct rte_node_mbuf_dynfield {
+#if RTE_NODE_MBUF_PERSISTENT_FIELDS_SIZE > 0
 	/**
 	 * Persistent mbuf region across nodes in graph walk
 	 *
@@ -78,6 +79,7 @@ typedef struct rte_node_mbuf_dynfield {
 	union {
 		uint8_t persistent_data[RTE_NODE_MBUF_PERSISTENT_FIELDS_SIZE];
 	};
+#endif
 	/**
 	 * Overloadable mbuf fields across graph walk. Fields which can change.
 	 *
-- 
2.48.1