IVGCVSW-4483 Remove boost::polymorphic_downcast

 * exchange boost::polymorphic_downcast with armnn::PolymorphicDowncast
 * remove unnecessary includes of boost::polymorphic_downcast

Signed-off-by: Jan Eilers <jan.eilers@arm.com>
Change-Id: Ie603fb82860fe05fee547dc78073230cc62b2e1f
diff --git a/src/backends/backendsCommon/WorkloadUtils.cpp b/src/backends/backendsCommon/WorkloadUtils.cpp
index bd5e81e..37915cf 100644
--- a/src/backends/backendsCommon/WorkloadUtils.cpp
+++ b/src/backends/backendsCommon/WorkloadUtils.cpp
@@ -7,6 +7,8 @@
 
 #include <armnn/Utils.hpp>
 
+#include <boost/numeric/conversion/cast.hpp>
+
 namespace armnn
 {