netlink: export netlink_diag.h header

The netlink_diag.h is in include/uapi/linux but not in the Kbuild necessary
to cause it to be exported by make headers_install.
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/Kbuild

@@ -261,6 +261,7 @@ header-y += net_dropmon.h
 header-y += net_tstamp.h
 header-y += netconf.h
 header-y += netdevice.h
+header-y += netlink_diag.h
 header-y += netfilter.h
 header-y += netfilter_arp.h
 header-y += netfilter_bridge.h