gpsd: PPS feature depends on NTPSHM feature

The PPS feature of gpsd depends on the NTP shared memory time hinting feature.

This patch caused the NTPSHM feature to be enabled whenever the PPS feature
is enabled. This should fix build failures such as the following.

  http://autobuild.buildroot.net/results/e58a9a9f884acb6834a8afe4a8a3d056dc509359/build-end.log



Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>