IVGCVSW-1804 : Add Subtraction layer types and placeholders

Change-Id: Ib9a477e5ce590df74ba05fece77258b9204f6523
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 802ca50..7890cdf 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -341,6 +341,8 @@
     src/armnn/layers/SoftmaxLayer.cpp
     src/armnn/layers/SplitterLayer.hpp
     src/armnn/layers/SplitterLayer.cpp
+    src/armnn/layers/SubtractionLayer.cpp
+    src/armnn/layers/SubtractionLayer.hpp
     src/armnn/Half.hpp
     src/armnn/InternalTypes.hpp
     src/armnn/InternalTypes.cpp