cpp: Add const-overload of UserID::Signature::operator<

lang/cpp/src/key.h, lang/cpp/src/key.cpp (UserID::Signature::operator<):
Add const-overload. Deprecate non-const overload.

--

This fixes compilation on FreeBSD with clang 10.

lang/cpp/src/key.cpp

@@ -866,6 +866,12 @@ UserID::Signature::Signature(const shared_gpgme_key_t &k, gpgme_user_id_t u, gpg
 }
 
 bool UserID::Signature::operator<(const Signature &other)
+{
+    // kept for binary compatibility
+    return static_cast<const UserID::Signature *>(this)->operator<(other);
+}
+
+bool UserID::Signature::operator<(const Signature &other) const
 {
     // based on cmp_signodes() in g10/keylist.c
 

lang/cpp/src/key.h

@@ -468,7 +468,9 @@ public:
     }
 
     /*! Defines a canonical sort order for signatures of the same user ID. */
-    bool operator<(const Signature &other);
+    bool operator<(const Signature &other) const;
+
+    GPGMEPP_DEPRECATED bool operator<(const Signature &other);
 
     bool isNull() const
     {