value: fix build without jsoncpp

[cho-m]

Seen in Homebrew (Homebrew/homebrew-core#101711) where we haven't added optional jsoncpp features

Issue was that checkSignature and decrypt were added inside the #ifdef OPENDHT_JSONCPP, so they weren't getting compiled resulting in

Undefined symbols for architecture x86_64:
    "dht::Value::checkSignature()", referenced from:
        dht::SecureDht::checkValue(std::__1::shared_ptr<dht::Value> const&) in securedht.cpp.o
        std::__1::__function::__func<dht::SecureDht::secureType(dht::ValueType&&)::$_3, std::__1::allocator<dht::SecureDht::secureType(dht::ValueType&&)::$_3>, bool (dht::Hash<20ul>, std::__1::shared_ptr<dht::Value>&, dht::Hash<20ul> const&, dht::SockAddr const&)>::operator()(dht::Hash<20ul>&&, std::__1::shared_ptr<dht::Value>&, dht::Hash<20ul> const&, dht::SockAddr const&) in securedht.cpp.o
        std::__1::__function::__func<dht::SecureDht::secureType(dht::ValueType&&)::$_4, std::__1::allocator<dht::SecureDht::secureType(dht::ValueType&&)::$_4>, bool (dht::Hash<20ul>, std::__1::shared_ptr<dht::Value> const&, std::__1::shared_ptr<dht::Value>&, dht::Hash<20ul> const&, dht::SockAddr const&)>::operator()(dht::Hash<20ul>&&, std::__1::shared_ptr<dht::Value> const&, std::__1::shared_ptr<dht::Value>&, dht::Hash<20ul> const&, dht::SockAddr const&) in securedht.cpp.o
    "dht::Value::decrypt(dht::crypto::PrivateKey const&)", referenced from:
        dht::SecureDht::checkValue(std::__1::shared_ptr<dht::Value> const&) in securedht.cpp.o
  ld: symbol(s) not found for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  make[2]: *** [libopendht.2.4.4.dylib] Error 1
  make[1]: *** [CMakeFiles/opendht.dir/all] Error 2
  make[1]: *** Waiting for unfinished jobs....

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Building Homebrew formula for v2.4.4 with the patch worked for me.