Update godot-cpp

Updates the godot-cpp submodule to a version which supports universal macOS
builds.

- We now need to include the standard string in terminal.cpp.
- Clean-up .gitmodules while we're at it.

addons/godot_xterm/native/src/terminal.cpp

@@ -8,6 +8,7 @@
 #include <ResourceLoader.hpp>
 #include <Theme.hpp>
 #include <algorithm>
+#include <string>
 #include <xkbcommon/xkbcommon-keysyms.h>
 
 // For _populate_key_list(), see below.