x11/nvidia-driver: unbreak the build of two legacy driver versions

• Limit the scope of the K&R prototype fix to versions which actually have that `nvlink_freebsd.c' file
• Adjust the version check for `lib/libnvidia-wayland-client.so' which had first appeared in 515.43.04 beta release and add a comment on the PLIST_FILES usage there

Reported by: pkg-fallout