build: Add totem-pl-playlist.c to plparser_sources

The source file totem-pl-playlist.c is not being parsed by g-ir-scanner
because it is missing in instrospection_sources. This will implicitly
add it to that variable.

https://bugzilla.gnome.org/show_bug.cgi?id=707642
plparse: Add missing direction annotations to TotemPlPlaylist

https://bugzilla.gnome.org/show_bug.cgi?id=707642