Bluetooth: Mesh: Add missing select for POLL

The Bluetooth Mesh code depends on k_poll, so it should have the
appropriate select statement in its Kconfig.

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

Author: jhedberg

subsys/bluetooth/mesh/Kconfig

@@ -6,6 +6,7 @@
 menuconfig BT_MESH
 	bool "Bluetooth Mesh support"
 	depends on BT_OBSERVER && BT_BROADCASTER
+	select POLL
 	help
 	  This option enables Bluetooth Mesh support. The specific
 	  features that are available may depend on other features