Additional tweak for Mantis #2887 and follow-up to r10181: when modifying OF_COLLIDE, need to call obj_set_flags for collision code support

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10974 387891d4-d844-0410-90c0-e4c51a9137d3