IVGCVSW-1803 : add Ref Subtraction layer

Change-Id: I4c019d626f9369245eca6d549bbe7a28e141f198
diff --git a/Android.mk b/Android.mk
index a164535..796b4d8 100644
--- a/Android.mk
+++ b/Android.mk
@@ -128,11 +128,15 @@
         src/armnn/backends/RefWorkloads/Multiplication.cpp \
         src/armnn/backends/RefWorkloads/RefMultiplicationUint8Workload.cpp \
         src/armnn/backends/RefWorkloads/RefBaseConstantWorkload.cpp \
-        src/armnn/backends/RefWorkloads/RefAdditionUint8Workload.cpp \
         src/armnn/backends/RefWorkloads/RefResizeBilinearFloat32Workload.cpp \
         src/armnn/backends/RefWorkloads/RefBatchNormalizationFloat32Workload.cpp \
         src/armnn/backends/RefWorkloads/Broadcast.cpp \
         src/armnn/backends/RefWorkloads/Addition.cpp \
+        src/armnn/backends/RefWorkloads/RefAdditionUint8Workload.cpp \
+        src/armnn/backends/RefWorkloads/RefAdditionFloat32Workload.cpp \
+        src/armnn/backends/RefWorkloads/Subtraction.cpp \
+        src/armnn/backends/RefWorkloads/RefSubtractionUint8Workload.cpp \
+        src/armnn/backends/RefWorkloads/RefSubtractionFloat32Workload.cpp \
         src/armnn/backends/RefWorkloads/RefFakeQuantizationFloat32Workload.cpp \
         src/armnn/backends/RefWorkloads/ResizeBilinear.cpp \
         src/armnn/backends/RefWorkloads/RefSoftmaxUint8Workload.cpp \
@@ -158,7 +162,6 @@
         src/armnn/backends/RefWorkloads/RefConstantUint8Workload.cpp \
         src/armnn/backends/RefWorkloads/RefConstantFloat32Workload.cpp \
         src/armnn/backends/RefWorkloads/Pooling2d.cpp \
-        src/armnn/backends/RefWorkloads/RefAdditionFloat32Workload.cpp \
         src/armnn/backends/RefWorkloads/RefMergerFloat32Workload.cpp \
         src/armnn/backends/RefWorkloads/RefFullyConnectedUint8Workload.cpp \
         src/armnn/backends/RefWorkloads/RefPermuteWorkload.cpp \