cmake: enable warnings for unused functions

Ever since commit 823c0e9c (Fix broken logic for attr cache invalidation,
2014-04-17), we have completely disabled warnings for unused functions. The only
comment that was added back then is about "annoying extra warnings" from Clang,
but in fact we shouldn't just ignore warnings about functions which aren't used
at all. Instead, the right thing would be to either only conditionally compile
functions that aren't used in all configurations or, alternatively, to remove
functions that aren't required at all.

As remaining instances of unused functions have been removed in the last two
commits, re-enable the warning.

CMakeLists.txt

@@ -230,7 +230,7 @@ ELSE ()
 	ENABLE_WARNINGS(declaration-after-statement)
 	ENABLE_WARNINGS(shift-count-overflow)
 	DISABLE_WARNINGS(unused-const-variable)
-	DISABLE_WARNINGS(unused-function)
+	ENABLE_WARNINGS(unused-function)
 	ENABLE_WARNINGS(format)
 	ENABLE_WARNINGS(format-security)
 	ENABLE_WARNINGS(int-conversion)