Cast const char* args from guile to char*.

Swig's guile typemaps lack a const char* freearg typemap. When a function
argument is explicitly const char* Swig automatically discards the const
in the temporary's decl so freeing it isn't a problem, but if the function
arg is declared as something typedeffed to const char* the alias is used
in the temporary's decl causing an error about discarding the const
qualifier when it's time to free the temporary. Providing a const char*
freearg typemap works around the shortcoming.

gnucash/html/gnc-html.i

@@ -34,6 +34,9 @@
 #include <gnc-html.h>
 %}
 #if defined(SWIGGUILE)
+
+%typemap (freearg) const char* "if (must_free$argnum && $1) SWIG_free((char*)$1);";
+
 %{
 #include "guile-mappings.h"