Fix compile error on "%x."

Hi,
I am facing following error with %x.

```
$ cat sprintf_x_dot.c
#include <stdio.h>

int main(int argc, char **argv)
{
    char buf[128] = {};
    sprintf(buf, "%x.", 10);
    sprintf(buf, "%xa", 10);
    return 0;
}
$ ./qrintf-gcc sprintf_x_dot.c
sprintf_x_dot.c: In function 'main':
sprintf_x_dot.c:6:32: error: expected identifier before '(' token
     sprintf(buf, "%x.", 10);
                                ^
sprintf_x_dot.c:7:5: error: incompatible type for argument 1 of '_qrintf_finalize'
     sprintf(buf, "%xa", 10);
     ^
In file included from <command-line>:1:0:
././qrintf.h:58:19: note: expected 'qrintf_t' but argument is of type 'int'
 static inline int _qrintf_finalize(qrintf_t ctx)
```

This PR fixed qrintf-pp to support %x correctly.