[nrf fromlist] net: if: Add dummy member to anonymous union in net_if_addr

Without a member, the anonymous union ends up with different size in C
and C++, causing the following fields to have different offsets. Adding
a 1-byte dummy member ensures consistent layout across both compilers.

Upstream PR #: 90359

Signed-off-by: Adrian Gielniewski <[email protected]>

Author: adigie

include/zephyr/net/net_if.h

@@ -125,6 +125,7 @@ struct net_if_addr {
 			uint8_t acd_state;
 		};
 #endif /* CONFIG_NET_IPV4_ACD */
+		uint8_t _dummy;
 	};
 
 #if defined(CONFIG_NET_IPV6_DAD) || defined(CONFIG_NET_IPV4_ACD)