Fix warning for `cl_khr_fp64` in OpenCL 1.2

On Debian 7 with OpenCL 1.2 (`amd-libopencl1` from backports) I got the following warning each time I tried to use a PyOpenCL *ReductionKernel*. Adding a correct `#ifdef` solves the problem, this is why I added it everywhere.

```
$ PYOPENCL_CTX=0 PYOPENCL_COMPILER_OUTPUT=1 ./test_cl_array.py 
/.../venv/src/pyopencl/pyopencl/__init__.py:57: CompilerWarning: Built kernel retrieved from cache. Original from-source build had warnings:
Build on <pyopencl.Device 'Intel(R) Core(TM) i5 CPU       M 560  @ 2.67GHz' on 'AMD Accelerated Parallel Processing' at 0x13ee130> succeeded, but said:

"/tmp/OCLpZ9ajX.cl", line 6: warning: OpenCL extension is now part of core
          #pragma OPENCL EXTENSION cl_khr_fp64: enable
                                   ^

  warn(text, CompilerWarning)
/.../venv/src/pyopencl/pyopencl/__init__.py:57: CompilerWarning: Built kernel retrieved from cache. Original from-source build had warnings:
Build on <pyopencl.Device 'Intel(R) Core(TM) i5 CPU       M 560  @ 2.67GHz' on 'AMD Accelerated Parallel Processing' at 0x13ee130> succeeded, but said:

"/tmp/OCLTQ0J0B.cl", line 6: warning: OpenCL extension is now part of core
          #pragma OPENCL EXTENSION cl_khr_fp64: enable
                                   ^

  warn(text, CompilerWarning)
```

Related links:

- <http://www.khronos.org/message_boards/showthread.php/9054-cl_khr_fp64-becoming-an-quot-optional-core-feature-quot>
- <http://www.informit.com/articles/article.aspx?p=1732873&seqNum=13>