lib/upnp: use include path without upnp/ prefix and honor pkg-config CFLAGS

If libupnp is installed in a non-standard location we must rely on the
include path provided by $(pkg-config --cflags libupnp). Relative to the
path given from that command no prefix must be used to find the respective
files.

src/lib/upnp/ClientInit.hxx

@@ -22,7 +22,7 @@
 
 #include "check.h"
 
-#include <upnp/upnp.h>
+#include <upnp.h>
 
 void
 UpnpClientGlobalInit(UpnpClient_Handle &handle);