cJSON : Include only source file objects

- test.c can cause issues if there is another main function
- Closes https://github.com/espressif/esp-idf/issues/1883

components/json/component.mk

@@ -4,3 +4,4 @@
 COMPONENT_ADD_INCLUDEDIRS := cJSON
 COMPONENT_SRCDIRS := cJSON
 COMPONENT_SUBMODULES := cJSON
+COMPONENT_OBJS := cJSON/cJSON.o cJSON/cJSON_Utils.o