package/libupnp18: needs host-pkgconf

Otherwise build fails with:

```
configure.ac:630: error: possibly undefined macro: AC_MSG_ERROR
      If this token and others are legitimate, please use m4_pattern_allow.
```

Fixes:

  http://autobuild.buildroot.net/results/6d734e45ec923dfa7549e0480b3d8913fc29fddd/



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>