doom: fix types mismatch spotted by gcc 4.5.1
- some variables were defined as const but declared without const
- remove double declaration of mapnames* (already in .h)
- forceOldBsp is boolean, not int
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@27660 a1c6a512-1295-4272-9138-f99709370657