batman-adv: debugfs, avoid compiling for !DEBUG_FS

Normally the debugfs framework will return error pointer with -ENODEV
for function calls when DEBUG_FS is not set.

batman does not notice this error code and continues trying to create
debugfs files and executes more code. We can avoid this code execution
by disabling compiling debugfs.c when DEBUG_FS is not set.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>