uapi: fix to export linux/vm_sockets.h

A typo "header=y" was introduced by commit 7071cf7f
(uapi: add missing network related headers to kbuild).

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>