drivers: i3c: fix warning with cpp builds

When compiling with C++ enabled (CONFIG_CPP), add an unused member to
prevent an empty struct; this makes the struct size the same for both C
and C++.

Signed-off-by: Ryan McClelland <ryanmcclelland@meta.com>

Author: XenuIsWatching

include/zephyr/drivers/i3c.h

@@ -1189,9 +1189,16 @@ struct i3c_driver_config {
 
 	/** I3C Primary Controller Dynamic Address */
 	uint8_t primary_controller_da;
+#elif defined(CONFIG_CPP)
+       /* Empty struct has size 0 in C, size 1 in C++. Force them to be the same. */
+	uint8_t unused_cpp_size_compatibility;
 #endif
 };
 
+#if defined(CONFIG_CPP)
+BUILD_ASSERT(sizeof(struct i3c_driver_config) >= 1);
+#endif
+
 /**
  * This structure is common to all I3C drivers and is expected to be the first
  * element in the driver's struct driver_data declaration.