Fix header parsing for 64-bit CPUs

[virtulis]

I had trouble parsing some table headers (the fields would come out garbled or empty), and after finding a note from the original author that a 32-build should be used and confirming that helps I traced it to this single variable that is used for it's "sizeof" to advance the file pointer.

Making sure it's always 4 bytes helped. The actual value isn't used, as far as I can tell, but I didn't want to change more than needed.