Silence -Wunused-parameter

It happened in open_object() if no support for opening files with xdg-open
or equivalent was enabled.

src/desktop_util.cpp

@@ -106,6 +106,7 @@ bool open_object(const std::string& path_or_url)
 
 #else
 
+	(void)(path_or_url); // silence gcc's -Wunused-parameter
 	ERR_DU << "open_object(): unsupported platform\n";
 	return false;