[PATCH] node: fix C++ compatibility errors (option 1)

[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 defaulting to a having a 1-byte sized union rather than
zero-sized.

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

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
 lib/node/rte_node_mbuf_dynfield.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/node/rte_node_mbuf_dynfield.h b/lib/node/rte_node_mbuf_dynfield.h
index b77fdbb94f..35674ca263 100644
--- a/lib/node/rte_node_mbuf_dynfield.h
+++ b/lib/node/rte_node_mbuf_dynfield.h
@@ -26,12 +26,13 @@ extern "C" {

 #ifndef RTE_NODE_MBUF_PERSISTENT_FIELDS_SIZE
 /** Size of persistent mbuf fields */
-#define RTE_NODE_MBUF_PERSISTENT_FIELDS_SIZE          (0)
+#define RTE_NODE_MBUF_PERSISTENT_FIELDS_SIZE      (1)  /* Must be non-zero to avoid a zero-sized
+                                                        * union, which causes C++ compat issues*/
 #endif /* !RTE_NODE_MBUF_PERSISTENT_FIELDS_SIZE */

 #ifndef RTE_NODE_MBUF_OVERLOADABLE_FIELDS_SIZE
 /** Size of overloadable mbuf fields */
-#define RTE_NODE_MBUF_OVERLOADABLE_FIELDS_SIZE        (8)
+#define RTE_NODE_MBUF_OVERLOADABLE_FIELDS_SIZE   (8)
 #endif /* !RTE_NODE_MBUF_OVERLOADABLE_FIELDS_SIZE */

 /** Size of node mbuf dynamic field */
--
2.48.1