community/libcec: raspberry pi support

Build libcec with raspberry pi support

I needed a working cec-client on a rpi and the one available as community/libcec doesn't work on a rpi as stated in #7754 (closed)

I fear that my simple fix breaks libcec for other architectures/devices, but I have no idea how to enable rpi support conditionally. I'm willing to test building and running libcec on an rpi if someone has other suggestions on how to provide rpi support for libcec.