Also fixed a writable strings warning/error.
warning: ISO C++11 does not allow conversion from
      string literal to 'char *' [-Wwritable-strings]

Several were of this form and fixed as the compiler suggested:
openclwrapper.cpp:2411:33: error: non-constant-expression cannot be narrowed
      from type 'int' to 'size_t' (aka 'unsigned long') in initializer list
      [-Wc++11-narrowing]
    size_t local_work_size[] = {block_size};
                                ^~~~~~~~~~
openclwrapper.cpp:2411:33: note: insert an explicit cast to silence this issue
    size_t local_work_size[] = {block_size};
                                ^~~~~~~~~~
                                static_cast<size_t>( )

Should have low impact on other platforms/compilers. The change makes
the code more correct.