Add EXECUTORCH_CLIENTS for visibility in extension/export_util:export_util

Differential Revision: D69076842

Pull Request resolved: pytorch#8155

Author: YIWENX14

extension/export_util/TARGETS

@@ -12,6 +12,7 @@ runtime.python_library(
         "//bento/...",
         "//bento_kernels/...",
         "//executorch/...",
+        "@EXECUTORCH_CLIENTS",
     ],
     deps = [
         "//caffe2:torch",