fix: add missing direct include of <vector> in file_writer.h (#584)

## Summary
- `file_writer.h` uses `std::vector<int64_t>` as the return type of
`Writer::split_offsets()` but does not directly include `<vector>`,
relying on a transitive include
- Add the missing `<vector>` include to follow the include-what-you-use
principle

Co-authored-by: shangxinli <shangxinli@users.noreply.github.com>

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"