build: fix tarantool build failure on xcode 12.5

`VERSION` files in small subproject and in tarantool are treated as C++
standard library on a filesystem with case-insensitive names. So we have
to delete the root of tarantool project from `include_directories` in
tarantool CMake. Also we have to change `include_directories` in tarantool
CMake from the root of `small` project to `include` subfolder in `small`
project.
Closes #6076