qt: fix menus/playlist icon alias

Merged Johannes Kauffmann requested to merge JohannesKauffmann/vlc2:qt-playlist-svg into master

In 68fc976d, the filename itself was updated (adding the ic_ prefix). menus.cpp was also updated, but since menus.cpp depends on the alias name and not the actual filename, this was causing the following warnings:

qt.svg: Cannot open file ':/menu/ic_playlist.svg', because: No such file or directory

Thus, update the alias also.

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