package/wireguard-linux-compat: tweak kernel-headers < 5.6 logic

The reason why the external wireguard kernel module is not allowed with
kernel headers >= 5.6 is that wireguard is included in the upstream kernel
since 5.6 rather than some kind of (fixable) incompatibility issue.  Adjust
the comment to make that clear.

While we're at it, drop the redundant !5.6 dependency on the kernel headers
dependency comment.  If headers are older than 3.10, then they are also
older than 5.6, so the statement is redundant.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>