Fix API inconsistency in cupp11.hpp
Fix API inconsistency in cupp11.hpp

The function `CopyToAsync` has an optional event argument in the OpenCL version, which is used in CLBlast. This makes the code not compile at all if CUDA (through cupp11.hpp`) is used as backend. This issue was found by a CLBlast user and reported privately by email. This PR should fix that.
CNugteren/CLBlast,CNugteren/CLBlast,CNugteren/CLBlast