Use 'class' instead of 'struct' in a friend declaration to be consistent with the definition.

This caused a warning in MSVC.
diff --git a/include/fruit/component_function.h b/include/fruit/component_function.h
index a51d8e3..8819f2d 100644
--- a/include/fruit/component_function.h
+++ b/include/fruit/component_function.h
@@ -41,7 +41,7 @@
         fruit::Component<ComponentParams...> (*getComponent)(FormalArgs...),
         ActualArgs&&... args);
 
-    friend class fruit::impl::ComponentStorageEntry;
+    friend struct fruit::impl::ComponentStorageEntry;
 
 public:
     ComponentFunction(const ComponentFunction&) = default;