tests/amdgpu: add missing header to SOURCES

Fixes the tarball generation.

fixes: 9d133dd0 (tests/amdgpu: add uvd encode unit tests)
bug: https://bugs.freedesktop.org/show_bug.cgi?id=102391


Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>