Fix errors when compiling cpp project against libh2o

I was receiving the following errors when compiling, explicitly typecasting seemed to fix the issue and I now have a working h2o server:

```
In file included from vendor/h2o/include/h2o.h:42:
In file included from vendor/h2o/include/h2o/http1client.h:26:
vendor/h2o/include/h2o/memory.h:293:17: error: cannot initialize a variable of type 'const char *' with an lvalue of
      type 'const void *'
    const char *target = _target, *test = _test;
                ^        ~~~~~~~
vendor/h2o/include/h2o/memory.h:293:36: error: cannot initialize a variable of type 'const char *' with an lvalue of
      type 'const void *'
    const char *target = _target, *test = _test;
                                   ^      ~~~~~
2 errors generated.
```