hexen: Store minotaur start time as little endian.

The minotaur uses the first four bytes of the mobj_t args[] array to
store its spawn time; after a certain amount of time has passed the
minotaur self-destructs. But the level time was being copied from the
leveltime variable without any endian conversion taking place. For
compatibility with Vanilla Hexen savegames we need to store the start
time in little endian format.

This fixes the first issue noted in #477 (thanks Quasar).