RuntimeError: In function void opencl::fft_inplace

Using this arrayfire release (https://arrayfire.s3.amazonaws.com/3.8.0/ArrayFire-v3.8.0_Linux_x86_64.sh) on the opencl backend with the python api I get the following error.
```python
import arrayfire as af
af.set_backend('opencl')
random_signal = af.random.randn(100)
test = af.signal.fft(random_signal)
```
Error
```
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call last)
in
----> 1 test = af.signal.fft(random_signal)

~/miniconda3/envs/af/lib/python3.7/site-packages/arrayfire/signal.py in fft(signal, dim0, scale)
177
178 output = Array()
--> 179 safe_call(backend.get().af_fft(c_pointer(output.arr), signal.arr, c_double_t(scale), c_dim_t(dim0)))
180 return output
181

~/miniconda3/envs/af/lib/python3.7/site-packages/arrayfire/util.py in safe_call(af_error)
77 err_len = c_dim_t(0)
78 backend.get().af_get_last_error(c_pointer(err_str), c_pointer(err_len))
---> 79 raise RuntimeError(to_str(err_str))
80
81 def get_version():

RuntimeError: In function void opencl::fft_inplace(opencl::Array&, int, bool) [with T = cl_float2]
In file src/backend/opencl/fft.cpp:
```

I'm running Ubuntu 20.04 with Nvidia OpenCL Drivers. I have arrayfire debugging enabled and got the following output when the error occurred.

```
BUILD LOG
************************************************
:19:10: error: expected expression
(float2)(};
^
:891:1: error: expected '}'
^
:18:32: note: to match this '{'
__constant float2 twiddles[] = {
^
:890:2: error: expected ';' after top level declarator
}
^
;

************************************************
FFTGeneratedStockhamAction::compileKernels failed

```

Reproduce the Python example with ArrayFire 3.8.0, the OpenCL backend, and Nvidia OpenCL drivers. Start with af.signal.fft and the reported src/backend/opencl/fft.cpp path, then inspect the generated FFT kernel and build log; done means the example completes without the malformed-kernel RuntimeError.

索引モデルが issue の本文から書いたものです。