video_core: Enforce -Wunused-function

Stops us from merging code with unused functions in the future.

If something is invoked behind conditionally evaluated code in
a way that the language can't see it (e.g. preprocessor macros), the
potentially unused function should use [[maybe_unused]].

src/video_core/CMakeLists.txt

@@ -314,6 +314,7 @@ else()
         -Werror=shadow
         -Werror=switch
         -Werror=type-limits
+        -Werror=unused-function
         -Werror=unused-variable
 
         $<$<CXX_COMPILER_ID:GNU>:-Werror=class-memaccess>