Fix undefined reference to VariantPool::usage()

extras/tests/ResourceManager/StringBuilder.cpp

@@ -3,6 +3,7 @@
 // MIT License
 
 #include <ArduinoJson/Memory/StringBuilder.hpp>
+#include <ArduinoJson/Memory/VariantPoolImpl.hpp>
 #include <catch.hpp>
 
 #include "Allocators.hpp"

extras/tests/ResourceManager/saveString.cpp

@@ -3,6 +3,7 @@
 // MIT License
 
 #include <ArduinoJson/Memory/ResourceManager.hpp>
+#include <ArduinoJson/Memory/VariantPoolImpl.hpp>
 #include <ArduinoJson/Strings/StringAdapters.hpp>
 #include <catch.hpp>