6lowpan: add request for ipv6 module

The iphc module depends on CONFIG_IPV6, because it's not very useful to
build the module without IPv6 support. Recently an user reported about
issues for setting an IPv6 address to a 6LoWPAN interface. The issues
was solved by modprobe the ipv6 module before. To avoid such user issues
we try to request the ipv6 module when the 6LoWPAN module is loaded.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>