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>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>