COMPMID-1750: Add PriorBox operator to graph API.

Change-Id: I5d2ed5dcc342abff8124762f7bdee587cdf20032
diff --git a/arm_compute/graph/Types.h b/arm_compute/graph/Types.h
index 24c24d3..b6803c8 100644
--- a/arm_compute/graph/Types.h
+++ b/arm_compute/graph/Types.h
@@ -51,6 +51,7 @@
 using arm_compute::PadStrideInfo;
 using arm_compute::PoolingLayerInfo;
 using arm_compute::PoolingType;
+using arm_compute::PriorBoxLayerInfo;
 using arm_compute::DimensionRoundingType;
 using arm_compute::InterpolationPolicy;
 
@@ -141,6 +142,7 @@
     PadLayer,
     PermuteLayer,
     PoolingLayer,
+    PriorBoxLayer,
     ReorgLayer,
     ReshapeLayer,
     ResizeLayer,