package/batman-adv: fix compile with BR2_PACKAGE_BATMAN_ADV_BATMAN_V disabled

package/batman-adv: fix compile with BR2_PACKAGE_BATMAN_ADV_BATMAN_V disabled

The given 'CONFIG_BATMAN_ADV_BATMAN_V=' is enough to trigger the wrong
code compile path in net/batman-adv/bat_v.h missing the static inline
dummy implementations.

Fixes:

  ERROR: modpost: "batadv_v_mesh_free" [.../build/batman-adv-2021.0/net/batman-adv/batman-adv.ko] undefined!
  ERROR: modpost: "batadv_v_mesh_init" [.../build/batman-adv-2021.0/net/batman-adv/batman-adv.ko] undefined!
  ERROR: modpost: "batadv_v_hardif_init" [.../build/batman-adv-2021.0/net/batman-adv/batman-adv.ko] undefined!
  ERROR: modpost: "batadv_v_init" [.../build/batman-adv-2021.0/net/batman-adv/batman-adv.ko] undefined!

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
[yann.morin.1998@free.fr:
  - move all conditional options together
  - slight cleanup/reorganise
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>