Fix to occupancy API call

[mondus]

Issue discovered in FLAME GPU2 PR #1334
and replicated with Jitify 2 where SimpleTest fails on Google Colab and Sheffield HPC with older drivers.

Jitify uses JITIFY_USE_CONTEXT_INDEPENDENT_LOADING to define jitify2::CudaFunction as either a CUkernel or a CUfunction however ConfiguredKernel::configure_1d_max_occupancy unconditionally casts this as a CUFunction (see L2566).

Fix is to conditionally obtain the function form the kernel using cuKernelGetFunction where JITIFY_USE_CONTEXT_INDEPENDENT_LOADING is defined.