iscsitarget: resolve build error if CONFIG_IPV6 config is not enabled

The element skc_v6_daddr (in struct sock_common) is defined in
kernel source only when CONFIG_IPV6 is enabled. Hence, access
sk_v6_daddr element (i.e __sk_common.skc_v6_daddr) only when CONFIG_IPV6
is defined; to fix below error in world build:

-- snip --
/home/jenkins/oe/world/shr-core/tmp-glibc/work-shared/qemux86/kernel-source/include/net/sock.h:330:33: error: 'struct sock_common' has no member named 'skc_v6_daddr'
-- CUT --

Signed-off-by: Jagadeesh Krishnanjanappa <jkrishnanjanappa@mvista.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>