Remove const of char** variable in API signature

Initial problem was on pycangjie2 library, in order to downgrade cython
required version from 0.17 to 0.14 (default version of  Ubuntu 12.04
    LTS), we needed to remove const attribute in signature. In cangjie_get_radical function, signature of radical was const char**, we changed it to char** and modify body of the function to comply with that.