Bluetooth: Mesh: Fix build error when LPN is not enabled
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 <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

GiulianoFranchetto/zephyr,galak/zephyr,punitvara/zephyr,finikorg/zephyr,mbolivar/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,punitvara/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr,zephyriot/zephyr,zephyriot/zephyr,explora26/zephyr,ldts/zephyr,ldts/zephyr,galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,mbolivar/zephyr,explora26/zephyr,punitvara/zephyr,explora26/zephyr,mbolivar/zephyr,explora26/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,kraj/zephyr,finikorg/zephyr,mbolivar/zephyr,finikorg/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,nashif/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyriot/zephyr,kraj/zephyr,zephyriot/zephyr,kraj/zephyr,ldts/zephyr,nashif/zephyr,Vudentz/zephyr