fix: Add missing header file (#56)

## Description
There is a missing header for Constants.mm, which causes the build to
crash. This PR fixes that by adding the header file

### Type of change
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] Documentation update (improves or adds clarity to existing
documentation)

### Tested on
- [ ] iOS
- [ ] Android

### Testing instructions
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Include setup details if necessary. -->

### Screenshots
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### Related issues
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### Checklist
- [ ] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have updated the documentation accordingly
- [ ] My changes generate no new warnings

### Additional notes
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reviewers might need to understand this PR. -->

Author: chmjkb

ios/RnExecutorch/utils/Constants.h

@@ -0,0 +1,8 @@
+#ifndef Constants_h
+#define Constants_h
+
+#include <map>
+
+extern const std::unordered_map<int, std::string> cocoLabelsMap;
+
+#endif /* Constants_h */