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>

include/uapi/linux/Kbuild

@@ -427,7 +427,7 @@ header-y += virtio_net.h
 header-y += virtio_pci.h
 header-y += virtio_ring.h
 header-y += virtio_rng.h
-header=y += vm_sockets.h
+header-y += vm_sockets.h
 header-y += vt.h
 header-y += wait.h
 header-y += wanrouter.h