[mlir] Fix nested codeowners for vector dialect
Due to how CODEOWNERS works, if a file matches two rules, the later rule will take precedence.
Because of this, /mlir/include/mlir/Dialect/Vector rules were not matching against /mlir/include/mlir/Dialect/Vector/IR .
This patch adds users in the directory rules, to the finer grained rules, which I'm guessing was the intended effect that they expected when they were added.
