[mlir][nvvm] Add missing include to llvm-config.h. (#83998)

This is another follow-up of #83004. `NVVM/Target.cpp` uses the macro
`MLIR_NVPTXCOMPILER_ENABLED`, which is defined in `llvm-config.h` but
did not include that file, yielding a warning when compiled with
`-Wundef`. This PR adds the include.

~~This is another follow-up of #83004, which made the same change for
`MLIR_CUDA_CONVERSIONS_ENABLED`. As the previous PR, this PR commit
exposes mentioned CMake variable through `mlir-config.h` and uses the
macro that is introduced with the same name. This replaces the macro
`MLIR_NVPTXCOMPILER_ENABLED`, which the CMake files previously defined
manually.~~