library.json: List LittleFS dependency

Like Hash, this is technically an external library, though it's supplied
by the framework.  Since we include the SPIFFSEditor by default, it is
necessary for the CI system to compile on ESP8266es.

Author: willmmiles

library.json

@@ -39,6 +39,10 @@
     {
       "name": "Hash",
       "platforms": "espressif8266"
+    },
+    {
+      "name": "LittleFS",
+      "platforms": "espressif8266"
     }
   ]
 }