rpmsg: rename .3d source to PascalCase to match wire 0.9.0

rpmsg_endpoint_info.3d was tracked lowercase in git, but wire 0.9.0's
file_base capitalizes the first letter so the generated dune.inc
targets Rpmsg_endpoint_info.3d. macOS case-insensitive filesystems
hid the mismatch; on case-sensitive filesystems dune fails with
\"No rule found for c/Rpmsg_endpoint_info.3d\" under `dune build -p rpmsg`.