c/shaders: Add back credit accidentally removed during shader overhaul
The updated versions of these shaders was added before
the originals were removed, so rename detection didn't trigger,
and d0617c04d5cb0cd4fd1f7e5bfabad54ff4540bd3 accidentally
removed credit.
Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2236>