ncrack fails to build in Debian unstable due to -Werror=implicit-function-declaration

Related to https://github.com/nmap/ncrack/issues/134

```
xmalloc.c: In function ‘xasprintf’:
xmalloc.c:114:13: error: implicit declaration of function ‘vasprintf’; did you mean ‘xasprintf’? [-Werror=implicit-function-declaration]
  114 |         i = vasprintf(ret, fmt, ap);
      |             ^~~~~~~~~
      |             xasprintf
make[2]: Leaving directory '/<<PKGBUILDDIR>>/nbase'
```
