netsupport: l2tp-ip: only depend on IPv6 if IPv6 support is enabled

netsupport: l2tp-ip: only depend on IPv6 if IPv6 support is enabled

Before r45593 kmod-l2tp-ip did not depend on kmod-ipv6.
With r45593 support for L2TP IPv6 encapsulation was added and
included in the kmod-l2tp-ip package. This change also
added the dependency to kmod-ipv6 to kmod-l2tp-ip, regardless
of whether the user chose to generally include IPv6 support
or not.
Change this so L2TP over IPv6 and the resulting dependency
to kmod-ipv6 is only included in kmod-l2tp-ip if IPv6 support
is enabled.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45612 3c298f89-4303-0410-b956-a3cf2f4a3e73