Update on "Introduce public MergedDataMap"

Add public merged data map. Module can use this to resolve multiple named data maps.

Differential Revision: [D83527299](https://our.internmc.facebook.com/intern/diff/D83527299/)

[ghstack-poisoned]

Author: lucylq

extension/named_data_map/merged_data_map.cpp

@@ -10,6 +10,7 @@
 #include <executorch/runtime/core/data_loader.h>
 
 #include <vector>
+#include <unordered_map>
 
 using executorch::aten::string_view;
 using executorch::ET_RUNTIME_NAMESPACE::NamedDataMap;