netlink: specs: allow uapi-header in genetlink

Chuck wanted to put the UAPI header in linux/net/ which seems
reasonable, allow genetlink families to choose the location.
It doesn't really matter for non-C-like languages.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>