Fix -Woverloaded-virtual warning in Schema by adding using declaration

Schema::Equals(const Schema&) hides StructType::Equals(const Type&),
triggering Clang's -Woverloaded-virtual warning. Add 'using StructType::Equals'
to bring the base class overload into scope alongside the Schema-specific one.

Author: WZhuo

src/iceberg/schema.h

@@ -187,6 +187,7 @@ class ICEBERG_EXPORT Schema : public StructType {
   friend bool operator==(const Schema& lhs, const Schema& rhs) { return lhs.Equals(rhs); }
 
  private:
+  using StructType::Equals;
   /// \brief Compare two schemas for equality.
   bool Equals(const Schema& other) const;