IVGCVSW-2125 : Consolidate backend registries into one

* adding missing include <functional> into BackendRegistry.hpp

Change-Id: I7a4ba867a856c90a3099a782ff41b8a5fb9a9efb
diff --git a/src/backends/backendsCommon/BackendRegistry.hpp b/src/backends/backendsCommon/BackendRegistry.hpp
index 2a52e24..82e59d9 100644
--- a/src/backends/backendsCommon/BackendRegistry.hpp
+++ b/src/backends/backendsCommon/BackendRegistry.hpp
@@ -9,6 +9,7 @@
 
 #include <memory>
 #include <unordered_map>
+#include <functional>
 
 namespace armnn
 {
@@ -57,4 +58,4 @@
 
 BackendRegistry& BackendRegistryInstance();
 
-} // namespace armnn
\ No newline at end of file
+} // namespace armnn