lib/upnp/ContentDirectoryService: work around GCC 5 bug

src/lib/upnp/ContentDirectoryService.cxx

@@ -71,7 +71,9 @@ ContentDirectoryService::getSearchCapabilities(UpnpClient_Handle hdl) const
 	const char *s = ixmlwrap::getFirstElementValue(response.get(),
 						       "SearchCaps");
 	if (s == nullptr || *s == 0)
-		return {};
+		/* we could just "return {}" here, but GCC 5 doesn't
+		   understand that */
+		return std::forward_list<std::string>();
 
 	return SplitString(s, ',', false);
 }