Error Compiling on Linux
When trying to compile this on Arch Linux, I am getting this error
```
Documents/xmrig-nvidia/build on  master [?] 
➜ make
[  2%] Building NVCC (Device) object CMakeFiles/xmrig-cuda.dir/src/nvidia/xmrig-cuda_generated_cuda_extra.cu.o
nvcc fatal   : Unsupported gpu architecture 'compute_20'
CMake Error at xmrig-cuda_generated_cuda_extra.cu.o.Release.cmake:222 (message):
  Error generating
  /home/thomas/Documents/xmrig-nvidia/build/CMakeFiles/xmrig-cuda.dir/src/nvidia/./xmrig-cuda_generated_cuda_extra.cu.o


make[2]: *** [CMakeFiles/xmrig-cuda.dir/build.make:72: CMakeFiles/xmrig-cuda.dir/src/nvidia/xmrig-cuda_generated_cuda_extra.cu.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:67: CMakeFiles/xmrig-cuda.dir/all] Error 2
make: *** [Makefile:84: all] Error 2
```

If it helps at all, I am running a GTX 1080ti, i7-4790, and I did install Cuda 9
```
Documents/xmrig-nvidia/build on  master [?] 
➜ nvcc --version
nvcc: NVIDIA (R) Cuda compiler driver
Copyright (c) 2005-2017 NVIDIA Corporation
Built on Fri_Sep__1_21:08:03_CDT_2017
Cuda compilation tools, release 9.0, V9.0.176
```