package/linuxptp: fix build with new headers

net_tstamp.h in recent kernel versions requires time.h for clockid_t.

Fixes:
  http://autobuild.buildroot.net/results/93903e8abfeaa7df0b6525033fa63e29f2c9e7ae



Signed-off-by: Artem Panfilov <panfilov.artyom@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>