Correctly expect the return value of rindex(const char*) to be of type

Correctly expect the return value of rindex(const char*) to be of type
'const char*' to make the code build on gcc-4.4.

The C++ spec overloads string fucntions like strtsr and rindex so that
rindex(char *) returns 'char*' and rindex(const char*) returns 'const
char*'.
Without this patch you get an "invalid conversion from ‘const char*’ to
‘char*’" error on gcc-4.4