use const propagation for GPU sinc function in PPPMForceComputeGPU
use const propagation for GPU sinc function in PPPMForceComputeGPU

refs #1534

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue