writable-string error on OSX with 10.4.8

MDEV-17609 introduced a change to client/mysql.cc which is causing an error with clang on OSX. We discovered this in our CI building 10.4.8 from source.

The error we are seeing on OSX is:

mariadb-10.4.8/client/mysql.cc:2671:21: error: ISO C++11 does not allow conversion from string literal to 'char *' [-Werror,-Wwritable-strings]

  rl_readline_name= "mysql";

It looks like this was fixed in 10.5 with a merge from 10.4. branch but I can't find the actual change included in the 10.4. branch.
https://github.com/MariaDB/server/commit/edef6a007428599fd249815f1dc59a02428090f4

 rl_readline_name= (char *) "mysql";

The cast to `(char *)` is all that is needed to remove this error from OSX builds.

I will push a PR to github shortly with this fix for the 10.4 branch.