net/freerdp3: Allow build RDPECAM with CAIRO

Enabling the "RDPECAM" option requires one of the "SWSCALE" or "CAIRO"
options (not just "SWSCALE") - replace "RADIO" with "SINGLE" to force
one of them on and avoid overcomplicating the logic.

PR: 294662
Tested by: Quentin Thébault <quentin.thebault@defenso.fr>
Sponsored by: UNIS Labs