Bluetooth: Mesh: Fix build error when LPN is not enabled

Some versions of gcc do not seem to compile out the inaccessible code
in this case and instead give the following error:

subsys/bluetooth/host/mesh/transport.c:419: undefined reference to
`bt_mesh_lpn_poll'

This happens at least when building samples/bluetooth/mesh for
native_posix on Fedora 28.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>