'const' error with Pd>=0.49

[umlaeute]

since Pd>=0.49, the t_symbol holds a const char* (rather than a char*).

This breaks compilation, as csoundSetStringChannel expects a char* pointer, when we only really have a const char* (in csoundapi_tilde.c:736).

A quick and dirty fix is to just cast away the constness (assuming that the 3rd argument of csoundSetStringChannel is really handled as a const string)