[OpenMP][NVPTX] Fixed missing variables for CUDA free compilation in NVPTX plugin

`CU_EVENT_DEFAULT` is defined in CUDA header. It should be added to
`openmp/libomptarget/plugins/cuda/dynamic_cuda/cuda.h` for CUDA free build.

Reviewed By: ronlieb

Differential Revision: https://reviews.llvm.org/D108878