Added empty function for ws public api

Author: Mika Tervonen

source/6LoWPAN/ws/ws_empty_functions.c

@@ -89,6 +89,15 @@ int ws_management_fhss_timing_configure(
     (void)fhss_bc_dwell_interval;
     return -1;
 }
+
+int ws_management_fhss_channel_function_configure(
+    int8_t interface_id,
+    uint8_t channel_function)
+{
+    (void)interface_id;
+    (void)channel_function;
+    return -1;
+}
 /* ### test api ### */
 int ws_test_pan_size_set(int8_t interface_id, uint16_t pan_size)
 {