kldxref: fix bootstrapping on Linux with Clang 16

The glibc fts_open() callback type does not have the second const
qualifier and it appears that Clang 16 errors by default for mismatched
function pointer types. Add an ifdef to handle this case.

usr.sbin/kldxref/kldxref.c

@@ -748,7 +748,11 @@ usage(void)
 }
 
 static int
+#ifdef __GLIBC__
+compare(const FTSENT **a, const FTSENT **b)
+#else
 compare(const FTSENT *const *a, const FTSENT *const *b)
+#endif
 {
 
 	if ((*a)->fts_info == FTS_D && (*b)->fts_info != FTS_D)