lxc: update comment about the headers >= 3.0 dependency

lxc: update comment about the headers >= 3.0 dependency

Since commit 604095fe9bea925f34ea05a5d80efbc3e9ba16f2 ("libcap: add
patch to fix build issue with old kernel headers"), libcap builds fine
with headers < 3.0, so it is no longer the reason why lxc needs
headers >= 3.0.

However, lxc uses setns(), which is only available since kernel 3.0,
so we simply update the comment next to the dependency so that it is
accurate.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>