Fix Clang -Wsign-conversion warning in lcm-gen C++
Fix Clang -Wsign-conversion warning in lcm-gen C++

When building C++ code generated by lcm-gen using Clang, a warning about
implicit conversion that changes signedness is
triggered (-Wsign-conversion). This patch fixes the warning by making
the conversion explicit, using static_cast<int64_t>().

lcm-proj/lcm,bluesquall/lcm,lcm-proj/lcm,lcm-proj/lcm,bluesquall/lcm,lcm-proj/lcm,lcm-proj/lcm,lcm-proj/lcm,bluesquall/lcm,lcm-proj/lcm,bluesquall/lcm,bluesquall/lcm,bluesquall/lcm,lcm-proj/lcm,bluesquall/lcm