Fix for ROCm (and CUDA?) breakage - 201029

The following commit breaks Eigen for ROCm (and probably CUDA too) with the following error

https://gitlab.com/libeigen/eigen/-/commit/e265f7ed8e59c26e15f2c35162c6b8da1c5d594f

```

Building HIPCC object test/CMakeFiles/gpu_basic.dir/gpu_basic_generated_gpu_basic.cu.o
In file included from /home/rocm-user/eigen/test/gpu_basic.cu:20:
In file included from /home/rocm-user/eigen/test/main.h:355:
In file included from /home/rocm-user/eigen/Eigen/QR:11:
In file included from /home/rocm-user/eigen/Eigen/Core:169:
/home/rocm-user/eigen/Eigen/src/Core/arch/Default/Half.h:825:76: error: use of undeclared identifier 'numext'; did you mean 'Eigen::numext'?
  return Eigen::half_impl::raw_uint16_to_half(__ldg(reinterpret_cast<const numext::uint16_t*>(ptr)));
                                                                           ^~~~~~
                                                                           Eigen::numext
/home/rocm-user/eigen/Eigen/src/Core/MathFunctions.h:968:11: note: 'Eigen::numext' declared here
namespace numext {
          ^
1 error generated when compiling for gfx900.
CMake Error at gpu_basic_generated_gpu_basic.cu.o.cmake:192 (message):
  Error generating file
  /home/rocm-user/eigen/build/test/CMakeFiles/gpu_basic.dir//./gpu_basic_generated_gpu_basic.cu.o

test/CMakeFiles/gpu_basic.dir/build.make:63: recipe for target 'test/CMakeFiles/gpu_basic.dir/gpu_basic_generated_gpu_basic.cu.o' failed
make[3]: *** [test/CMakeFiles/gpu_basic.dir/gpu_basic_generated_gpu_basic.cu.o] Error 1
CMakeFiles/Makefile2:16611: recipe for target 'test/CMakeFiles/gpu_basic.dir/all' failed
make[2]: *** [test/CMakeFiles/gpu_basic.dir/all] Error 2
CMakeFiles/Makefile2:16618: recipe for target 'test/CMakeFiles/gpu_basic.dir/rule' failed
make[1]: *** [test/CMakeFiles/gpu_basic.dir/rule] Error 2
Makefile:5401: recipe for target 'gpu_basic' failed
make: *** [gpu_basic] Error 2
```

The fix is in this commit is trivial. Please review and merge