Fix GCC_ARM linker error with some Thread builds (ARMmbed#1742)

Add HAVE_WS in c-file to fix: "Error: L6218E: Undefined symbol
DEVICE_MIN_SENS (referred from ws_neighbor_class.o)"

Author: Arto Kinnunen

source/6LoWPAN/ws/ws_neighbor_class.c

@@ -25,6 +25,8 @@
 #include "6LoWPAN/ws/ws_config.h"
 #include "6LoWPAN/ws/ws_neighbor_class.h"
 
+#ifdef HAVE_WS
+
 #define RSL_UNITITIALIZED 0x7fff
 #define TRACE_GROUP "wsne"
 
@@ -143,3 +145,5 @@ void ws_neighbor_class_rsl_out_calculate(ws_neighbor_class_entry_t *ws_neighbor,
     return;
 }
 
+#endif /* HAVE_WS */
+