MSVC build fix
MSVC build fix

MSVC doesn't seem to align uint32_t blkOffset : 24; to 3 bytes, despite
the pack(1) pragma.

FranciscoDA/OpenApoc,FranciscoDA/OpenApoc,FranciscoDA/OpenApoc,steveschnepp/OpenApoc,pmprog/OpenApoc,Istrebitel/OpenApoc,steveschnepp/OpenApoc,pmprog/OpenApoc,Istrebitel/OpenApoc