cmake: add missing compile definition

The RING_ACCEL macro was not defined when compiling the unit tests with
the JAMI_VIDEO_ACCEL option enabled, which caused a heap buffer overflow
when running ut_media_decoder and resulted in the test failing in CI
with a "free(): invalid pointer" error.

GitLab: #1124
Change-Id: I333502a570224d9dd52a2b4e5e8ebe86d96e41a6