add a missing #include

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9714 387891d4-d844-0410-90c0-e4c51a9137d3
this is required for strict const-correctness

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9715 387891d4-d844-0410-90c0-e4c51a9137d3