Only run warn.py's main function if it is the main program.

Test: run warn.py --byproject build.log
Change-Id: I155cce8c952d6d8a02c8e1809bf61a46c847c26e

tools/warn.py

@@ -2333,10 +2333,14 @@ def dump_csv():
   print '{},,{}'.format(total, 'All warnings')
 
 
-##### Main function starts here. #########################
-
-parse_input_file()
-if args.gencsv:
+def main():
+  parse_input_file()
+  if args.gencsv:
     dump_csv()
-else:
+  else:
     dump_html()
+
+
+# Run main function if warn.py is the main program.
+if __name__ == '__main__':
+  main()