feat: expose mlir_lexer.py to bazel overlay

Author: Wheest

utils/bazel/llvm-project-overlay/mlir/BUILD.bazel

@@ -27,6 +27,7 @@ exports_files([
     "LICENSE.TXT",
     "run_lit.sh",
     "utils/lldb-scripts/mlirDataFormatters.py",
+    "utils/pygments/mlir_lexer.py",
     "utils/textmate/mlir.json",
 ])