[CMake] add fix for emscripten/web builds.

[enetheru]

A recently submitted issue i godot-cpp highlighted the fact that for emscripten builds need a specific inclusion to occur prior to the project command , but after the updating of the godot-cpp sources.

https://github.com/godotengine/godot-cpp/issues/1830

I'm creating this issue so that I remember to update the template.

It's essentially a one liner

set(CMAKE_PROJECT_<my project name goes here>_INCLUDE ${godot-cpp_SOURCE_DIR}/cmake/emsdkHack.cmake)