Use THRUST_DEVICE_SYSTEM instead of THRUST_DEVICE_BACKEND

Thrust 1.7 deprecates THRUST_DEVICE_BACKEND in place of
THRUST_DEVICE_SYSTEM. THRUST_DEVICE_SYSTEM is used in the main GooFit
Makefile. Change it in pipipi0DPFit example to be consistent and remove
warnings from Thrust.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>