cmake: enable warnings for unused const variables

Together with the warnings for unused warnings, we always had warnings
for unused constant variables disabled since commit 823c0e9c (Fix
broken logic for attr cache invalidation, 2014-04-17). As we have now
fixed all occurrences of such variables, we can safely enable those
warnings again.

CMakeLists.txt

@@ -229,7 +229,7 @@ ELSE ()
 	ENABLE_WARNINGS(strict-prototypes)
 	ENABLE_WARNINGS(declaration-after-statement)
 	ENABLE_WARNINGS(shift-count-overflow)
-	DISABLE_WARNINGS(unused-const-variable)
+	ENABLE_WARNINGS(unused-const-variable)
 	ENABLE_WARNINGS(unused-function)
 	ENABLE_WARNINGS(format)
 	ENABLE_WARNINGS(format-security)