gpsd: fix Python 2.5 build failure

The platform.linux_distribution method was introduced in Python 2.6. Some
of the Buildroot autobuilders still use Python 2.5, which is causing build
failures such as the following.

  http://autobuild.buildroot.net/results/045f1f69bac170d8e75cb4952a2e5b4e85a677b8/build-end.log



This patch removes the linux distribution check from the gpsd SConstruct file.

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