cmake: Enforce -Wunused-function code-base wide

src/CMakeLists.txt

@@ -69,6 +69,7 @@ else()
         -Werror=reorder
         -Werror=switch
         -Werror=uninitialized
+        -Werror=unused-function
         -Werror=unused-result
         -Werror=unused-variable
         -Wextra

src/video_core/CMakeLists.txt

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