locking/rtmutex: Use struct keyword in kernel-doc comment

Add the "struct" keyword to prevent a kernel-doc warning:

rtmutex_common.h:67: warning: cannot understand function prototype: 'struct rt_wake_q_head '

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250111063040.910763-1-rdunlap@infradead.org

Author: rddunlap

kernel/locking/rtmutex_common.h

@@ -59,8 +59,8 @@ struct rt_mutex_waiter {
 };
 
 /**
- * rt_wake_q_head - Wrapper around regular wake_q_head to support
- *		    "sleeping" spinlocks on RT
+ * struct rt_wake_q_head - Wrapper around regular wake_q_head to support
+ *			   "sleeping" spinlocks on RT
  * @head:		The regular wake_q_head for sleeping lock variants
  * @rtlock_task:	Task pointer for RT lock (spin/rwlock) wakeups
  */