Update godot-cpp submodule

- We now need to include `<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.

addons/godot_xterm/native/thirdparty/godot-cpp

@@ -1 +1 @@
-Subproject commit 26d4df4e174124e1174c624d770661b1394fcf5f
+Subproject commit f9f6767722e17a1d690f7ce975e1e7f8065b61e2