Fix missing include for std::unique_ptr.

This template is defined in <memory>, but it was not included.

Somehow this was not causing problems with older versions of GCC,
probably because it was transitively included by something else.

However, this caused an error with GCC 12 that is stricter in this
regard.

Signed-off-by: Oleg Girko <ol@infoserver.lv>

src/threshold.cpp

@@ -2,6 +2,8 @@
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <memory>
+
 #include "threshold.hpp"
 
 #include "schemes.hpp"
@@ -296,4 +298,4 @@ namespace bls {
     bool Threshold::Verify(const G1Element& pubKey, const Bytes& vecMessage, const G2Element& signature) {
         return pThresholdScheme->Verify(pubKey, vecMessage, signature);
     }
-}
+}