Fix for MusicInfoScanner to honor "Part of Compilation" flag in music media.

Background: I have several handmade/virtual Albums, which are made up out of miscellanous music files. All items have valid meta information and the "Part of Compilation" flag set. Within XBMC Music database, each track shows up as an idividual Album, the "Part of Compilation" flag is not honored.

The fix will only take effect, if ALL tracks of one album have the "Part of Compilation" flag set. In this case, the album is made to a compilation, even if the tracks overlap.