Use symbol visibility attributes

Hi again, this is part of another port to Meson. Just like libbluray, I've added symbol visibility attributes to the library headers to replace libtool's -export-symbols-regex option. This time, however, I've kept things simpler by adding a new header at dvdread/attributes.h. As before, I've checked to make sure that the list of exported symbols is identical across all the major OSes.