Fix __clang_cuda_math_forward_declares.h

Recent change from `#if !defined(__CUDA__)` to `#if !__CUDA__` caused
regression on ROCm 3.5 since there is `#define __CUDA__`
before inclusion of the header file, which causes `#if !__CUDA__`
to be invalid.

Change `#if !__CUDA__` back to `#if !defined(__CUDA__)` for backward
compatibility.