pkg: iotivity: Hide put_light function if not needed

Without this change it fails to build from source:
   client_switch.c:80:1:\
   error: ‘put_light’ defined but not used [-Werror=unused-function]

At least with BOARD=arduino-mega2560, now all iotivity examples are building.

Bug: https://github.com/RIOT-OS/RIOT/issues/6241
Change-Id: I6b2af5ab6b8bf20e848dcd30d580990006c950e7
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>

examples/iotivity_examples/client_switch/client_switch.c

@@ -76,6 +76,7 @@ stop_observe(void *data)
     return DONE;
 }
 
+#ifdef BUTTON_GPIO
 static void
 put_light(oc_client_response_t *data)
 {
@@ -87,6 +88,7 @@ put_light(oc_client_response_t *data)
         PRINT("client_oic: PUT response code %d\n", data->code);
     }
 }
+#endif
 
 static void
 observe_light(oc_client_response_t *data)