Fix ambiguous auto variable on gcc 7.5

[smnogar]

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I'm running humble on an older version of embedded linux thats stuck with gcc 7.5. For some reason it didn't like this auto declaration. Could we get this pushed into rolling and humble branches?

This should resolve https://github.com/ros2/rclcpp/issues/1963

[clalancette]

@smnogar The linters are unhappy with the length of the line. If you fix that, we can run CI on this and get it in.

[fujitatomoya]

@smnogar would you address comments above? thanks!

[clalancette]

Since there has been no movement on this in a long time, I'm going to close this out. Feel free to respond to the comments and reopen if you are still interested in getting this in, thanks.