write_queue: Fix Doxygen comment

Change-Id: Ib2508411ae46e2456466beaae63d3f401e34d0d4
diff --git a/src/core/write_queue.c b/src/core/write_queue.c
index ffa7704..8fb73a6 100644
--- a/src/core/write_queue.c
+++ b/src/core/write_queue.c
@@ -147,7 +147,7 @@
 	queue->bfd.when &= ~OSMO_FD_WRITE;
 }
 
-/* Update write queue length & drop excess messages.
+/*! Update write queue length & drop excess messages.
  * \param[in] queue linked list header of message queue
  * \param[in] len new max. wqueue length
  * \returns Number of messages dropped.