The operator< should be const, so it can be applied to const Signatures.

This fixes a build failure in security/libkleo.

diff --git lang/cpp/src/key.cpp lang/cpp/src/key.cpp

index e536d080..7411e9e4 100644

--- lang/cpp/src/key.cpp

+++ lang/cpp/src/key.cpp

@@ -865,7 +865,7 @@ UserID::Signature::Signature(const shared_gpgme_key_t &k, gpgme_user_id_t u, gpg

 {

 }

-bool UserID::Signature::operator<(const Signature &other)

+bool UserID::Signature::operator<(const Signature &other) const

 {

     // based on cmp_signodes() in g10/keylist.c

diff --git lang/cpp/src/key.h lang/cpp/src/key.h

index a7931672..57001a03 100644

--- lang/cpp/src/key.h

+++ lang/cpp/src/key.h

@@ -468,7 +468,7 @@ public:

     }

     /*! Defines a canonical sort order for signatures of the same user ID. */

-    bool operator<(const Signature &other);

+    bool operator<(const Signature &other) const;

     bool isNull() const

     {