[3.0] contrib: dvdread: do not use btn_it and its parent as packed structures

The 4x 12 bits packing is making the bitfield packing messy and incompatible with (non)MS packing in dvdnav. In the end the structures are not read as packed structures so we don't need to keep them packed.

Upstream patch proposed at libdvdread!42

(cherry picked from commit 94db2f10)