Allow use of argument param files for J2ObjC scripts.

--
MOS_MIGRATED_REVID=100964806
diff --git a/tools/objc/j2objc_dead_code_pruner.py b/tools/objc/j2objc_dead_code_pruner.py
index 546a9ef..2245778 100755
--- a/tools/objc/j2objc_dead_code_pruner.py
+++ b/tools/objc/j2objc_dead_code_pruner.py
@@ -215,7 +215,7 @@
 
 
 if __name__ == '__main__':
-  parser = argparse.ArgumentParser()
+  parser = argparse.ArgumentParser(fromfile_prefix_chars='@')
   parser.add_argument(
       '--input_files',
       required=True,