batman-adv: statically print gateway table header

To make it easier to search through the code it is better to print static
strings directly instead of using format strings printing constants.

This was addressed in a previous patch, but the Gateway table header
was not updated accordingly.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Reviewed-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>