IVGCVSW-6310 Update ICustomAllocator and add GetDefaultAllocator function to backends

Signed-off-by: Francis Murtagh <francis.murtagh@arm.com>
Change-Id: If2879e8a82692285e7fa3f4d09abd608ca28de12
diff --git a/src/backends/reference/RefBackend.hpp b/src/backends/reference/RefBackend.hpp
index 4d4aba9..6114ce6 100644
--- a/src/backends/reference/RefBackend.hpp
+++ b/src/backends/reference/RefBackend.hpp
@@ -60,6 +60,8 @@
     {
         return cpuRefCapabilities;
     };
+
+    std::unique_ptr<ICustomAllocator> GetDefaultAllocator() const override;
 };
 
 } // namespace armnn