locking/rtmutex: Use the '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>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/20250307232717.1759087-2-boqun.feng@gmail.com