grff wrapper returns warnings when parsing the ip-link.8.in file.

How to reproduce:
$ man --warnings ip-link > /dev/null
`R' is a string (producing the registered sign), not a macro.
[...]
Signed-off-by: Thomas Faivre <thomas.faivre@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>