mkclean: Silence some bogus warnings

[mkver]

Currently mandatory elements like SeekPreRoll that are physically absent in the input file and only assumed because they are mandatory frequently lead to bogus warnings like "The SeekPreRoll element at -1 is not part of profile 'matroska v2', skipping".

PS: There is actually another issue here: If one remuxes a version two file, the same warning appears, although SeekPreRoll is only compatible with Matroska v4 and Webm. For Matroska v1-v3 one shouldn't assume SeekPreRoll at all.

[robUx4]

If the EBML_MasterCheckContext() reports a bogus error then it should be fixed there (in libebml2). Also if the element is not part of Matroska v2 and in such a file then it's an issue in the file. It's normal to report that it's not going to be there (it may be bogus).