Use "our" instead of "my" for zephyrStartup hook

All our other hooks seem to be defined using "our", and I don't seem to be able
to use the zephyrStartup hook without this change.
http://perldoc.perl.org/functions/our.html does claim the scoping should be the
same (though it also seems to suggest that only "our" gives a package variable
instead of something only in the current lexical scope...).