device: Update SYS_INIT_NAMED() to be C++ compatible

In #84394, `struct init_entry` was modified to remove an unneeded union.
The `SYS_INIT_NAMED()` macro was adjusted accordingly, but is no longer
C++ compatible due to the partial designated initializer.

Add an explicit value (NULL) for the other field (`dev`) in that struct.

Signed-off-by: Tristan Honscheid <[email protected]>

Author: tristan-google

include/zephyr/init.h

@@ -166,7 +166,7 @@ struct init_entry {
 #define SYS_INIT_NAMED(name, init_fn_, level, prio)                                       \
 	static const Z_DECL_ALIGN(struct init_entry)                                      \
 		Z_INIT_ENTRY_SECTION(level, prio, 0) __used __noasan                      \
-		Z_INIT_ENTRY_NAME(name) = {.init_fn = (init_fn_)}                         \
+		Z_INIT_ENTRY_NAME(name) = {.init_fn = (init_fn_), .dev = NULL}            \
 
 /** @} */