meson: fix build failure if orc is enabled but none of its users are
Potential fix for #778 (closed), where meson configure fails if orc is enabled but audiofx, deinterlace, videobox, and videomixer are all disabled.
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Potential fix for #778 (closed), where meson configure fails if orc is enabled but audiofx, deinterlace, videobox, and videomixer are all disabled.