use class declarations since these contain SCP_string

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11042 387891d4-d844-0410-90c0-e4c51a9137d3
VC6 compatibility

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11043 387891d4-d844-0410-90c0-e4c51a9137d3