[NV 2716223] `adjacent_difference` generates a warning (warning: calling a __host__ function from a __host__ __device__ function) with the OpenMP backend

Using `thrust::adjacent_difference` with `thrust::omp::par` execution policy produces the warning

```
thrust/system/detail/generic/adjacent_difference.inl(73): warning: calling a __host__ function from a __host__ __device__ function is not allowed
detected during:
instantiation of "OutputIterator thrust::system::detail::generic::adjacent_difference(thrust::execution_policy &, InputIterator, InputIterator, OutputIterator, BinaryFunction) [with DerivedPolicy=thrust::system::omp::detail::par_t, InputIterator=__gnu_cxx::__normal_iterator>>, OutputIterator=__gnu_cxx::__normal_iterator>>, BinaryFunction=thrust::minus]"
```

when passing `std::vector` iterators.

Minimal example:
```
#include
#include
#include

#include

int main(){
std::vector a{0};
std::vector b{1};

thrust::adjacent_difference(thrust::omp::par, a.begin(), a.end(), b.begin());
auto k = thrust::min(a.begin(), b.begin());
thrust::sort(thrust::omp::par, a.begin(), a.end());
}
```

Code is compiled with `nvcc main.cu -Xcompiler "-fopenmp" -o main`.
Cuda compilation tools, release 10.1, V10.1.243
The latest thrust version from Github is used.

NVBug 2716223