Try using clGetExtensionFunctionAddress. There's no guarantee that extension functions are exported in library files, and such a method of accessing them is defined as non-portable.
9.2 Getting OpenCL API Extension Function
Pointers
clGetExtensionFunctionAddress may not be queried for core (non-extension) functions in OpenCL. For functions that are queryable with clGetExtensionFunctionAddress, implementations may choose to also export those functions statically from the object libraries implementing those functions. However, portable applications cannot rely on this behavior.
That said, I have no idea what ATI's policy is regarding exporting extensions, so I don't know if it's intentionally not exported.