clang-tidy: Enable readability-convert-member-functions-to-static

This check finds non-static member functions that can be made
static because the functions don’t use this.
This check also triggers readability-static-accessed-through
-instance check as we are trying to access a static member
function through an instance.

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I6c06c61e8f199247e74aca3443741683e3853274
diff --git a/src/item_updater.hpp b/src/item_updater.hpp
index 2bfd39c..bc8930e 100644
--- a/src/item_updater.hpp
+++ b/src/item_updater.hpp
@@ -181,7 +181,7 @@
     void syncToLatestImage();
 
     /** @brief Invoke the activation via DBus */
-    void invokeActivation(const std::unique_ptr<Activation>& activation);
+    static void invokeActivation(const std::unique_ptr<Activation>& activation);
 
     /** @brief Callback function for interfaces added signal.
      *