IVGCVSW-1964 : replace optional biases with home-grown Optional

!android-nn-driver:151788

Change-Id: Ibdc41d09b8df05e7a0360dcb8a060860dfb1bd99
diff --git a/src/backends/neon/workloads/NeonDepthwiseConvolutionBaseWorkload.hpp b/src/backends/neon/workloads/NeonDepthwiseConvolutionBaseWorkload.hpp
index 982992a..ffee508 100644
--- a/src/backends/neon/workloads/NeonDepthwiseConvolutionBaseWorkload.hpp
+++ b/src/backends/neon/workloads/NeonDepthwiseConvolutionBaseWorkload.hpp
@@ -7,8 +7,6 @@
 
 #include <backends/neon/workloads/NeonWorkloadUtils.hpp>
 
-#include <boost/optional.hpp>
-
 namespace armnn
 {
 
@@ -16,6 +14,6 @@
                                                              const TensorInfo& output,
                                                              const DepthwiseConvolution2dDescriptor& descriptor,
                                                              const TensorInfo& weights,
-                                                             const boost::optional<TensorInfo>& biases);
+                                                             const Optional<TensorInfo>& biases);
 
 } // namespace armnn