IVGCVSW-4246 Clean build Layers with -Wextra

Change-Id: I649cd2304fb0040164763d31a12fc77c6c3bed87
Signed-off-by: Derek Lamberti <derek.lamberti@arm.com>
diff --git a/src/armnn/Network.cpp b/src/armnn/Network.cpp
index 3a2dc4e..1406160 100644
--- a/src/armnn/Network.cpp
+++ b/src/armnn/Network.cpp
@@ -545,7 +545,8 @@
                                                             OutputSlot& slot,
                                                             TensorHandleFactoryRegistry& registry)
 {
-   return ITensorHandleFactory::DeferredFactoryId;
+    boost::ignore_unused(backends, slot, registry);
+    return ITensorHandleFactory::DeferredFactoryId;
 }
 
 // For all handle factories supported on the source backend, we wish to find the one which requires the fewest copies