[mlir][Vector] Fix -Wunused-function in VectorEmulateNarrowType.cpp (NFC)

/data/llvm-project/mlir/lib/Dialect/Vector/Transforms/VectorEmulateNarrowType.cpp:229:21: error: unused function 'operator<<' [-Werror,-Wunused-function]
static raw_ostream &operator<<(raw_ostream &os,
                    ^
1 error generated.