net: add net_tstamp.h to headers_install

include/linux/net_tstamp.h is userspace API for hardware time stamping
of network packets. It should be exported to userspace.

Signed-off-by: Jie Zhang <jie.zhang@analog.com>
Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Patrick Ohly <patrick.ohly@gmx.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>