Cast index argument to cl_uint in kernel::get_arg_info()

This casts the 'index' argument in kernel::get_arg_info() to cl_uint
in order to match the signature for clGetKernelArgInfo(). This fixes
the "conversion from 'size_t' to 'cl_uint', possible loss of data"
compiler warning.

include/boost/compute/kernel.hpp

@@ -188,7 +188,9 @@ public:
     template<class T>
     T get_arg_info(size_t index, cl_kernel_arg_info info) const
     {
-        return detail::get_object_info<T>(clGetKernelArgInfo, m_kernel, info, index);
+        return detail::get_object_info<T>(
+            clGetKernelArgInfo, m_kernel, info, static_cast<cl_uint>(index)
+        );
     }
 
     /// \overload