Fix Steam overlay for Linux
Was getting some build and loading failures when trying to build overlay for linux.
This MR fixes them and makes that overlay works.
EMU_EXPERIMENTAL_BUILD
doesn't build on Linux and it's not actually needed for overlay on Linux unlike on Windows.
Only note that need to start game with LD_PRELOAD=libsteam_api.so ./game
as otherwise our lib might get loaded later so our symbols wouldn't be used.