bluedroid: Move fixed_queue.h to implementation of hci_layer.c

Avoids pulling types like list_node into the public interface.

Closes https://github.com/espressif/esp-idf/issues/948

components/bt/bluedroid/hci/hci_layer.c

@@ -32,6 +32,7 @@
 #include "alarm.h"
 #include "thread.h"
 #include "mutex.h"
+#include "fixed_queue.h"
 
 typedef struct {
     uint16_t opcode;

components/bt/bluedroid/hci/include/hci_layer.h

@@ -21,7 +21,6 @@
 
 #include "bt_types.h"
 #include "allocator.h"
-#include "fixed_queue.h"
 #include "osi.h"
 #include "future.h"
 ///// LEGACY DEFINITIONS /////