queue: queue_generate_id() is not necessary for export.

src/queue.c

@@ -20,7 +20,10 @@
 #include "queue.h"
 #include "song.h"
 
-unsigned
+/**
+ * Generate a non-existing id number.
+ */
+static unsigned
 queue_generate_id(const struct queue *queue)
 {
 	static unsigned cur = (unsigned)-1;

src/queue.h

@@ -230,12 +230,6 @@ void
 queue_finish(struct queue *queue);
 
 /**
- * Generate a non-existing id number.
- */
-unsigned
-queue_generate_id(const struct queue *queue);
-
-/**
  * Returns the order number following the specified one.  This takes
  * end of queue and "repeat" mode into account.
  *