libnm: belatedly expose nm_ethtool_optname_is_feature() in libnm
Also, plan right away to backport this symbol all the way back to
1.14.8. As such, we only need to add it once, with the right linker
But still, the symbols first appears on a major release 1.20.0.
(cherry picked from commit a4c14895)
(cherry picked from commit a7322eaa)