Add missing `.cuh` files to package data

!461 (merged) added cuda.cuh to include/pystencils_runtime, but forgot to list .cuh files in the setuptools package-data config. As a consequence, the cuh file was not being installed properly, and #include errors ensued.

requested review from @zy69guqi

assigned to @da15siwa

approved this merge request

mentioned in commit 1117a24b

merged