[boost] [serialization] assert missing

The header 'boost/serialization/item_version_type.hpp' seems to be missing an include on boost/assert.hpp (or cassert) that was causing some compilation failures in the graph_parallel tests (under GCC-4.4.3 on GNU/Linux x86_64). The test case brings this in by including serialization/list.hpp. My first thought was to simply stick an appropriate #include in the item_version_type. Would there be any problems committing this change? Andrew