libbase: Adding missing vprintf function.

libbase: Adding missing vprintf function.

Fixes #8.

```
int vprintf(const char *format, va_list ap);

The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equivalent to
the functions printf(), fprintf(), sprintf(), snprintf(), respectively, except
that  they  are  called  with  a  va_list instead of a variable number of
arguments.
```