add #ifdef around UPnP setting things to fix build with --disable-upnp

Discovered a build problem on my Gentoo box while updating to latest master (please forgive the german system locale/messages):

    CPP     xbmc/cores/paplayer/SPCCodec.o
    AR      xbmc/cores/paplayer/paplayer.a
    LD      xbmc.bin
    xbmc/settings/settings.a(Settings.o): In function `CSettings::Clear()':
    Settings.cpp:(.text+0x43da): undefined reference to `CUPnPSettings::Get()'
    Settings.cpp:(.text+0x43e2): undefined reference to `CUPnPSettings::Clear()'
    collect2: Fehler: ld gab 1 als Ende-Status zurück
    make: *** [xbmc.bin] Fehler 1

This happens when supplying "--disable-upnp" to configure (which in turn is done by USE="-upnp" by the ebuild). https://github.com/xbmc/xbmc/commit/ead4525c45142369bd2e1897e8c5b030fd34f37e seems to have introduced the problem.

The PR fixes that by adding #ifdef HAS_UPNP around the (new) UPnP things in xbmc/settings/Settings.cpp. Build and function tested on Linux and working fine.