opencl: Fix type of parameter for clGetContextInfo

CL_CONTEXT_NUM_DEVICES expects a cl_uint.

Passing size_t results in a wrong value for numDevices on hosts where
sizeof(cl_uint) != sizeof(size_t). This results in errors like these:

  Tesseract Open Source OCR Engine v3.05.00dev with Leptonica
  OpenCL error code is -44 at   when clCreateKernel kernel_HistogramRectAllChannels .
  OpenCL error code is -44 at   when clCreateKernel kernel_HistogramRectAllChannelsReduction .
  OpenCL error code is -48 at   when clSetKernelArg imageBuffer .
  ...

Signed-off-by: Stefan Weil <sw@weilnetz.de>

opencl/openclwrapper.cpp

@@ -595,7 +595,7 @@ int OpenclDevice::CompileKernelFile( GPUEnv *gpuInfo, const char *buildOption )
     const char *source;
     size_t source_size[1];
     int b_error, binary_status, binaryExisted, idx;
-    size_t numDevices;
+    cl_uint numDevices;
     cl_device_id *mpArryDevsID;
     FILE *fd, *fd1;
     const char* filename = "kernel.cl";