compilation against gcc-4.6

Compilation against gcc-4.6 fails with the message:
/builddir/build/BUILD/mscore-0.9.6.3/mscore/mscore/exportxml.cpp:2332:59: error: invalid conversion from 'const SLine*' to 'SLine*' [-fpermissive]

See [1] for the full log. Failure occurs in both 0.9.6.3 and 1.0. The attached patch (which is against 1.0) resolves the issue. Please consider applying to the trunk.

Orcan

[1] http://koji.fedoraproject.org/koji/getfile?taskID=2795763&name=build.log