batman-adv: move batadv_slide_own_bcast_window to bat_iv_ogm.c

batadv_slide_own_bcast_window() is used only in bat_iv_ogm.c
and it is currently touching only batman_iv specific
attributes.

Move it into bat_iv_ogm.c and make it static.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>