Fix truncated __OPENMP_NVPTX__ preprocessor condition

I don't have a test case for this but noticed this warning when including system headers with `-I` rather than `-isystem`.

```
In file included from <built-in>:1:
In file included from /nix/store/jq6bpm0xmhnbffhs5rkxq4n88g5xi2zg-clang-wrapper-11.0.1/resource-root/include/__clang_cuda_runtime_wrapper.h:157:
/nix/store/jq6bpm0xmhnbffhs5rkxq4n88g5xi2zg-clang-wrapper-11.0.1/resource-root/include/__clang_cuda_math.h:39:25: warning: extra tokens at end of #ifdef directive [-Wextra-tokens]
                        ^
```

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D95299

(cherry picked from commit c49142e4)