Implement hook_theme to support theme_geonames_table()

I was trying out the Geonames "Simple City Search" example and I could not get it to work.

Looked over the code and I discovered that there was no implementation of theme_hook() to register the theme_geonames_table() function for Drupal 6.

The theme hook is also missing from 6.x-2.x-dev

The patch below inserts the implementation of hook_theme() right before the theme_geonames_table() function. Not sure if that's the best place for it, but I will leave it to the maintainers to move it to where they like it.