Update external/Makefile to remove object files.
To resolve the issue
@@ -118,6 +118,7 @@ ifneq ("$(wildcard $(IOTIVITY_BASE_DIR)/iotivity.built)","")
$(Q) TOPDIR="$(TOPDIR)" $(TOPDIR)/../external/iotivity/clean_iotivity.sh
$(Q) rm -rf $(IOTIVITY_BASE_DIR)/out
$(Q) rm -rf $(IOTIVITY_BASE_DIR)/extlibs/mbedtls/mbedtls
+ $(Q) rm -rf $(IOTIVITY_BASE_DIR)/resource/csdk/connectivity/lib/libcoap-4.1.1/*.o
$(Q) rm -rf $(IOTIVITY_BASE_DIR)/.sconsign.dblite
$(Q) rm -rf $(IOTIVITY_BASE_DIR)/.sconf_temp
endif
