IVGCVSW-4246 Clean build of backends with -Wextra

Change-Id: I9e8d5576b3ec04c871785d5f2f9545bf1136e59b
Signed-off-by: Derek Lamberti <derek.lamberti@arm.com>
diff --git a/src/backends/reference/workloads/Gather.cpp b/src/backends/reference/workloads/Gather.cpp
index c848a7c..5416855 100644
--- a/src/backends/reference/workloads/Gather.cpp
+++ b/src/backends/reference/workloads/Gather.cpp
@@ -9,6 +9,7 @@
 
 #include <backendsCommon/WorkloadData.hpp>
 
+#include <boost/core/ignore_unused.hpp>
 #include <boost/numeric/conversion/cast.hpp>
 
 namespace armnn
@@ -21,6 +22,7 @@
             const int32_t* indices,
             Encoder<float>& output)
 {
+    boost::ignore_unused(outputInfo);
     const TensorShape& paramsShape = paramsInfo.GetShape();
 
     unsigned int paramsProduct = 1;