Fix MPS test internal build.

Summary: Missing export_util dep in buck.

Differential Revision: D61491499

Author: shoumikhin

backends/apple/mps/TARGETS

@@ -94,6 +94,7 @@ runtime.python_test(
         "//caffe2:torch",
         "//executorch/examples/models:models",
         "//executorch/exir/tests:models",
+        "//executorch/extension/export_util:export_util",
         "//executorch/sdk:lib",
         "//executorch/sdk/bundled_program/serialize:lib",
         "fbsource//third-party/pypi/pytest:pytest",