net: ethtool: avoid rebuilds on UTS_RELEASE change

Currently, when you switch between branches or something like that and
rebuild, net/ethtool/ioctl.c has to be built again because it depends
on UTS_RELEASE.

By instead referencing a string variable stored in another object file,
this can be avoided.

Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20240220194244.2056384-1-jannh@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>