Const modifiers

This functions do not change class variables
Would be good to mark them as const, so
class variables are not changed by coincidence

Change-Id: Iea34f6d26dbd1bde813035160e07ff2a681989e6
diff --git a/verifier.cpp b/verifier.cpp
index e9d540c..23142c1 100644
--- a/verifier.cpp
+++ b/verifier.cpp
@@ -370,7 +370,7 @@
 }
 
 struct BNDeleter {
-  void operator()(BIGNUM* bn) {
+  void operator()(BIGNUM* bn) const {
     BN_free(bn);
   }
 };