"Mapping" a uint64_t to a uint32_t

I have a RNG function xorshift128plus that takes a Xorshift128PlusKey:

/** 
        * \brief Keys for scalar xorshift128. Must be non-zero.
        * These are modified by xorshift128plus.
        */
        struct Xorshift128PlusKey
        {
            uint64_t s1;
            uint64_t s2;
        };

        /** 
        * \brief Return a new 64-bit random number.
        */
        uint64_t xorshift128plus(Xorshift128PlusKey* key);

I would like to seed my RNG using rdtsc (processor time stamp). The problem is that the __rdtsc intrinsic under msvc returns a 64 bit unsigned integer and the seed must be a 32 bit unsigned integer. What is the best way to convert the rdtsc to a seed while preserving randomness. The conversion must be as fast as possible.

I can't use the std lib or boost. (It's for a game engine)