Fix processing of preprocessing flags. (#12029)

Fixes https://github.com/iree-org/iree/issues/12028
diff --git a/tests/compiler_driver/BUILD b/tests/compiler_driver/BUILD
index 91d73f8..858e0b8 100644
--- a/tests/compiler_driver/BUILD
+++ b/tests/compiler_driver/BUILD
@@ -20,6 +20,7 @@
         [
             "executable_benchmarks.mlir",
             "hal_executable.mlir",
+            "preprocessing_flags.mlir",
             "smoketest.mlir",
             "streams.mlir",
         ],