tipc: do not add socket.o to tipc-y twice

net/tipc/Makefile adds socket.o twice.

tipc-y	+= addr.o bcast.o bearer.o \
           core.o link.o discover.o msg.o  \
           name_distr.o  subscr.o monitor.o name_table.o net.o  \
           netlink.o netlink_compat.o node.o socket.o eth_media.o \
                                             ^^^^^^^^
           topsrv.o socket.o group.o trace.o
                    ^^^^^^^^

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>