IVGCVSW-1938: Move backend-specific source files to the corresponding backend

Change-Id: I558a9a007604afc55e536d877f8da7d0215cc9c3
diff --git a/src/backends/cl/CMakeLists.txt b/src/backends/cl/CMakeLists.txt
index 04da6dd..2f32081 100644
--- a/src/backends/cl/CMakeLists.txt
+++ b/src/backends/cl/CMakeLists.txt
@@ -15,6 +15,11 @@
 )
 
 if(ARMCOMPUTECL)
+    list(APPEND armnnClBackend_sources
+        OpenClTimer.cpp
+        OpenClTimer.hpp
+    )
+
     add_subdirectory(workloads)
     add_subdirectory(test)
 endif()