Fix compilation error: const GDExtensionStringPtr -> GDExtensionConst…

[zhehangd]

g++ 9.4.0 and cmake

Got these compilation errors:

godot-cpp/src/variant/char_string.cpp:356:115: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
  356 |  internal::gde_interface->string_operator_plus_eq_string((GDExtensionStringPtr)this, (const GDExtensionStringPtr)&p_str);

godot-cpp/src/classes/low_level.cpp:51:151: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
   51 | )internal::gde_interface->worker_thread_pool_add_native_task(_owner, p_func, p_userdata, p_high_priority, (const GDExtensionStringPtr)&p_description);

godot-cpp/src/classes/low_level.cpp:55:179: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
   55 | orker_thread_pool_add_native_group_task(_owner, p_func, p_userdata, p_elements, p_tasks, p_high_priority, (const GDExtensionStringPtr)&p_description);

We expect const void* here but const GDExtensionStringPtr means void* const. We should use GDExtensionConstStringPtr instead.

[akien-mga]

Thanks!