lib: os: change __printk_get_hook return type to printk_hook_fn_t

Not sure why this was returning a void * but it seems to make sense to
return the correct type instead.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

include/zephyr/sys/printk-hooks.h

@@ -33,6 +33,6 @@ void __printk_hook_install(printk_hook_fn_t fn);
  *
  * @return a function pointer or NULL if no hook is set
  */
-void *__printk_get_hook(void);
+printk_hook_fn_t __printk_get_hook(void);
 
 #endif /* ZEPHYR_INCLUDE_SYS_PRINTK_HOOKS_H_ */

lib/os/printk.c

@@ -60,7 +60,7 @@ void __printk_hook_install(printk_hook_fn_t fn)
 	_char_out = fn;
 }
 
-void *__printk_get_hook(void)
+printk_hook_fn_t __printk_get_hook(void)
 {
 	return _char_out;
 }