Remove duplicated uint typedef

Running the OpenCL test on AMD's ROCm platform results in

pystencils_tests/test_opencl.py::test_without_cuda
  /usr/lib/python3/dist-packages/pyopencl/cffi_cl.py:1516: CompilerWarning: Built kernel retrieved from cache. Original from-source build had warnings:
  Build on <pyopencl.Device 'gfx900' on 'AMD Accelerated Parallel Processing' at 0x34f0f50> succeeded, but said:
  
  In file included from /tmp/comgr-1f94f8/input/CompileCLSource:1:
  ./pystencils/pystencils/include/opencl_stdint.h:4:27: warning: redefinition of typedef 'uint' is a C11 feature
  typedef unsigned int      uint;
                            ^
  /data/jenkins_workspace/compute-rocm-rel-2.9/out/ubuntu-16.04/16.04/srctf/ocl_lc/drivers/opencl/library/amdgcn/headers/build/lnx64a/B_rel/<stdin>:52:22: note: previous definition is here
  typedef unsigned int uint;

According to the specification, the uint type is part of the OpenCL C builtin types (and has been since version 1.0), so this typedef is not needed. In Nvidia's OpenCL stack, it appears to be built into the compiler, while LLVM (and thus AMD) define it in a header file.