emotion: disable generic-vlc backend

* we don't have vlc in DEPENDS, but when it happens to be built before
  emotion, it causes undeterministic builds and unpackaged files
  emotion-1.7.3: emotion: Files/directories were installed but not
  shipped
    /usr/lib/emotion/utils/em_generic_vlc

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>