IVGCVSW-4210 Create a public API for the common backend files

 * Create a public API for the common backend files
 * Move OutputHandler to armnn internal
 * Remove unused headers

Signed-off-by: Matteo Martincigh <matteo.martincigh@arm.com>
Signed-off-by: Narumol Prangnawarat <narumol.prangnawarat@arm.com>
Change-Id: I3e86d908b021e3561befa9d45158d87d2cbb18c0
diff --git a/src/backends/backendsCommon/test/layerTests/GatherTestImpl.hpp b/src/backends/backendsCommon/test/layerTests/GatherTestImpl.hpp
index 33df179..b6a2dd1 100644
--- a/src/backends/backendsCommon/test/layerTests/GatherTestImpl.hpp
+++ b/src/backends/backendsCommon/test/layerTests/GatherTestImpl.hpp
@@ -9,7 +9,7 @@
 
 #include <Half.hpp>
 
-#include <backendsCommon/IBackendInternal.hpp>
+#include <armnn/backends/IBackendInternal.hpp>
 #include <backendsCommon/WorkloadFactory.hpp>
 
 LayerTestResult<float, 1> Gather1dParamsFloat32Test(