fs: Avoid an unused variable warning

This is very configuration-specific and only seems to happen on Windows
at the moment, which is kind of intentional from what when I discussed
the code with josteph.

src/filesystem.cpp

@@ -611,6 +611,9 @@ static bool is_path_relative_to_cwd(const std::string& str)
 void set_user_data_dir(std::string newprefdir)
 {
 	bool relative_ok = false;
+	// Not always but it can be unused in some configurations
+	UNUSED(relative_ok);
+
 #ifdef PREFERENCES_DIR
 	if(newprefdir.empty()) {
 		newprefdir = PREFERENCES_DIR;