Added include for size_t type definition.
Added include for size_t type definition.

Change-Id: Id5c456954c341d259c5699099c1fc9899277ad43

qust113/molcore,OpenChemistry/avogadrolibs,wadejong/avogadrolibs,cjh1/mongochemweb-avogadrolibs,wadejong/avogadrolibs,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,wadejong/avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,qust113/molcore,wadejong/avogadrolibs,cjh1/mongochemweb-avogadrolibs,OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,cjh1/mongochemweb-avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,cjh1/mongochemweb-avogadrolibs,OpenChemistry/avogadrolibs