linux-zigbee: add missing comment for thread dependency

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/linux-zigbee/Config.in

@@ -1,3 +1,6 @@
+comment "linux-zigbee needs a toolchain w/ threads"
+	depends on !BR2_TOOLCHAIN_HAS_THREADS
+
 config BR2_PACKAGE_LINUX_ZIGBEE
 	bool "linux-zigbee"
 	depends on BR2_USE_MMU # fork()