do not write prefix to header when just packing

[formtapez]

When using "armake2 pack " to just pack an ArmA-Mission to a .pbo, armake2 will write a prefix to the pbo-header which confuses pbo-extractors like Mikero DePbo. (it extracts in an additionally folder and confuses the GUI by letting the prefixname even corrupt other missions you extract afterwards)
It seems to be no problem for ArmA to load such missions, but i could sleep better if you could just leave that away when no $PBOPREFIX$ file exists.