COMPMID-1750: Add PriorBox operator to graph API.

Change-Id: I5d2ed5dcc342abff8124762f7bdee587cdf20032
diff --git a/arm_compute/graph/nodes/NodesFwd.h b/arm_compute/graph/nodes/NodesFwd.h
index 8d9bad3..f956b54 100644
--- a/arm_compute/graph/nodes/NodesFwd.h
+++ b/arm_compute/graph/nodes/NodesFwd.h
@@ -51,6 +51,7 @@
 class PadLayerNode;
 class PermuteLayerNode;
 class PoolingLayerNode;
+class PriorBoxLayerNode;
 class ReorgLayerNode;
 class ReshapeLayerNode;
 class ResizeLayerNode;