ipv6 addrconf: Remove IFA_GLOBAL definition from include/net/if_inet6.h.

This patches removes IFA_GLOBAL definition from linux/include/net/if_inet6.h
as it is unused.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>