batman-adv: use static string for table headers

Use a static string when showing table headers rather then
a nonsense parametric one with fixed arguments.

It is easier to grep and it does not need to be recomputed
at runtime each time.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
[sven@narfation.org: fix conflicts with current version]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>