Fix the build on some platforms by using an explicit cast for Univalue type

Summary: It appears that std::vector::size_type is not implicitely convertible to uint64_t or int64_t on on platforms , so cast it explicitely.

Test Plan:

./contrib/teamcity/build-configurations.py build-linux32
./contrib/teamcity/build-configurations.py build-native-osx

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: #bitcoin_abc, tyler-smith

Subscribers: tyler-smith

Differential Revision: https://reviews.bitcoinabc.org/D11244