IVGCVSW-1963: Add implementation classes for Cl, Neon and Ref backends

Change-Id: Ibb2efaf8b001a64a3f35eab2220990dc39889780
diff --git a/src/backends/reference/backend.mk b/src/backends/reference/backend.mk
index e5345c0..365faa6 100644
--- a/src/backends/reference/backend.mk
+++ b/src/backends/reference/backend.mk
@@ -8,6 +8,7 @@
 # file in the root of ArmNN
 
 BACKEND_SOURCES := \
+        RefBackend.cpp \
         RefLayerSupport.cpp \
         RefWorkloadFactory.cpp \
         workloads/Activation.cpp \