include: vlc_poll: fix documentation nesting

Because vlc_poll.h was parsed before vlc_thread and the group name was
also thread, every thread-related item was moved to the "Poll
implementations" list on the doxygen output.

Regression from 3f613a85f5.

include/vlc_poll.h

@@ -31,7 +31,7 @@
 
 /**
  * \ingroup os
- * \defgroup thread Poll implementations
+ * \defgroup thread_poll Poll implementations
  * @{
  * \file
  * Poll implementations
@@ -96,4 +96,6 @@ static inline int vlc_poll (struct pollfd *fds, unsigned nfds, int timeout)
 
 #endif
 
+/** @} */
+
 #endif /* !VLC_POLL_H_ */