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.

Change-Id: Ia0867db019db8e40e710bdd4025e030061f7a992
Signed-off-by: Pavithra Barithaya <pavithrabarithaya07@gmail.com>
diff --git a/activation.hpp b/activation.hpp
index 891e582..22e0445 100644
--- a/activation.hpp
+++ b/activation.hpp
@@ -351,7 +351,8 @@
      *
      * @return true if verification successful and false otherwise
      */
-    bool verifySignature(const fs::path& imageDir, const fs::path& confDir);
+    static bool verifySignature(const fs::path& imageDir,
+                                const fs::path& confDir);
 
     /** @brief Called when image verification fails. */
     void onVerifyFailed();