wireguard: needs 3.10+ kernel

wireguard: needs 3.10+ kernel

The dependency is actually only for the kernel module (and thus on the
runtime kernel version rather than kernel headers), but as we don't know the
runtime version in kconfig and the user space part doesn't make much sense
without the kernel module this will have to do.

[Peter: extend commit message, add comment to Config.in as suggested by
	Arnout, fix off-by-one in comment text]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>