[OpenMP] Use OPENMP_NVPTX instead of _OPENMP in complex wrapper headers.
This is very similar to 7f1e6fcff942, just fixing a left-over.
With this, it should be possible to use both, -x cuda and -fopenmp in the same invocation,
enabling to use both OpenMP, targeting CPU, and CUDA, targeting the GPU.
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D90415