add missing override modifier for _process()

[pseidemann]

fixes warning when following the tutorial.

In file included from src/register_types.cpp:9:
src/gdexample.h:22:8: warning: '_process' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  void _process(double delta);
       ^
godot-cpp/gen/include/godot_cpp/classes/node.hpp:277:15: note: overridden virtual function is here
        virtual void _process(double delta);
                     ^

[mhilbrunner]

Merged. Thanks and congrats on your first merged contribution!