fix: add missing direct include of <vector> in file_writer.h

file_writer.h uses std::vector directly (split_offsets return type)
but relies on a transitive include for it. Add the missing <vector>
include to follow the include-what-you-use principle.

Author: shangxinli

src/iceberg/file_writer.h

@@ -25,6 +25,7 @@
 #include <functional>
 #include <memory>
 #include <optional>
+#include <vector>
 
 #include "iceberg/arrow_c_data.h"
 #include "iceberg/file_format.h"