[libc] fix time () not found when no RT_USING_RTC defined

components/libc/compilers/armlibc/stubs.c

@@ -327,3 +327,12 @@ int fgetc(FILE *f)
     return -1;
 }
 #endif
+
+#ifndef RT_USING_RTC
+time_t time(time_t *t)
+{
+	time_t time_now = 0;
+	
+	return time_now;
+}
+#endif